Assessment of Selected 

Inorganic Constituents in Streams in the 

Central Arizona Basins Study Area, 

Arizona and Northern Mexico, through 1998

Water-Resources Investigations Report 03—4063

National Water-Quality Assessment Program

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY
Cover photograph courtesy of Arizona-Sonora Desert Museum, Tucson, Arizona
Assessment of Selected 

Inorganic Constituents in Streams in the 

Central Arizona Basins Study Area, 

Arizona and Northern Mexico, through 1998

National Water-Quality Assessment Program

Water-Resources Investigations Report 03—4063

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

By David W. Anning

Tucson, Arizona

2003
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

For additional information write to:

District Chief

U.S. Geological Survey

Water Resources Division

520 N. Park Avenue, Suite 221

Tucson, AZ 85719–5035

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225–0046

The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute 

endorsement by the U.S. Geological Survey.

Information about U.S. Geological Survey programs in Arizona is available online at http://az.water.usgs.gov.
FOREWORD

The U.S. Geological Survey (USGS) is committed to serve the Nation with accurate and timely scientific information 

that helps enhance and protect the overall quality of life, and facilitates effective management of water, biological, 

energy, and mineral resources. (http://www.usgs.gov/). Information on the quality of the Nation's water resources is 

of critical interest to the USGS because it is so integrally linked to the long-term availability of water that is clean and 

safe for drinking and recreation and that is suitable for industry, irrigation, and habitat for fish and wildlife. Escalating 

population growth and increasing demands for the multiple water uses make water availability, now measured in 

terms of quantity and quality, even more critical to the long-term sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program to support national, regional, 

and local information needs and decisions related to water-quality management and policy. (http://water.usgs.gov/

nawqa/). Shaped by and coordinated with ongoing efforts of other Federal, State, and local agencies, the NAWQA 

Program is designed to answer: What is the condition of our Nation's streams and ground water? How are the 

conditions changing over time? How do natural features and human activities affect the quality of streams and ground 

water, and where are those effects most pronounced? By combining information on water chemistry, physical 

characteristics, stream habitat, and aquatic life, the NAWQA Program aims to provide science-based insights for 

current and emerging water issues and priorities. NAWQA results can contribute to informed decisions that result in 

practical and effective water-resource management and strategies that protect and restore water quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than 50 of the Nation's 

most important river basins and aquifers, referred to as Study Units. (http://water.usgs.gov/nawqa/nawqamap.html). 

Collectively, these Study Units account for more than 60 percent of the overall water use and population served by 

public water supply, and are representative of the Nation's major hydrologic landscapes, priority ecological resources, 

and agricultural, urban, and natural sources of contamination. 

Each assessment is guided by a nationally consistent study design and methods of sampling and analysis. The 

assessments thereby build local knowledge about water-quality issues and trends in a particular stream or aquifer 

while providing an understanding of how and why water quality varies regionally and nationally. The consistent,

multi-scale approach helps to determine if certain types of water-quality issues are isolated or pervasive, and allows 

direct comparisons of how human activities and natural processes affect water quality and ecological health in the 

Nation's diverse geographic and environmental settings. Comprehensive assessments on pesticides, nutrients, volatile 

organic compounds, trace metals, and aquatic ecology are developed at the national scale through comparative 

analysis of the Study-Unit findings. (http://water.usgs.gov/nawqa/natsyn.html). 

The USGS places high value on the communication and dissemination of credible, timely, and relevant science so that 

the most recent and available knowledge about water resources can be applied in management and policy decisions. 

We hope this NAWQA publication will provide you the needed insights and information to meet your needs, and 

thereby foster increased awareness and involvement in the protection and restoration of our Nation's waters. 

The NAWQA Program recognizes that a national assessment by a single program cannot address all water-resource 

issues of interest. External coordination at all levels is critical for a fully integrated understanding of watersheds and 

for cost-effective management, regulation, and conservation of our Nation's water resources. The Program, therefore, 

depends extensively on the advice, cooperation, and information from other Federal, State, interstate, Tribal, and 

local agencies, non-government organizations, industry, academia, and other stakeholder groups. The assistance and 

suggestions of all are greatly appreciated.

Robert M. Hirsch

Associate Director for Water

Contents v

CONTENTS

Page

Abstract ................................................................................................................................................................ 1

Introduction .......................................................................................................................................................... 2

Purpose and Scope ...................................................................................................................................... 3

Description of the Central Arizona Basins Study Area .............................................................................. 3

Physiography...................................................................................................................................... 3

Climate ............................................................................................................................................... 3

Surface-Water Hydrology .................................................................................................................. 6

Stream-Property and Water-Chemistry Data ....................................................................................................... 7

Data Selection and Station Categorization.................................................................................................. 7

Distribution of Stream-Property and Water-Chemistry Data...................................................................... 9

Spatial distribution of stations............................................................................................................ 9

Temporal Distribution of Water-Quality Samples............................................................................. 14

Streamflow at the Time of Sampling ................................................................................................. 16

Assessment of Water Quality by Comparisons to Regulatory Standards ............................................................ 22

Stream temperature, pH, and dissolved oxygen.......................................................................................... 22

Dissolved solids and suspended sediment................................................................................................... 33

Nutrients...................................................................................................................................................... 34

Relation of Stream Properties and Water-Chemistry Constituent Concentrations 

to Natural and Human Factors ................................................................................................................ 35

Streamflow and Season............................................................................................................................... 35

Unregulated Perennial and Intermittent Reaches............................................................................... 36

Unregulated Urban Ephemeral Reaches ............................................................................................ 60

Regulated Reaches ............................................................................................................................. 60

Effluent-Dependent Reaches.............................................................................................................. 61

Upstream Conditions................................................................................................................................... 63

Temporal Trends ......................................................................................................................................... 71

Streamflow ......................................................................................................................................... 71

Water Quality..................................................................................................................................... 76

Relation of Stream Loads to Upstream Conditions.............................................................................................. 78

Dissolved Solids.......................................................................................................................................... 78

Nutrients...................................................................................................................................................... 84

Summary and Conclusions................................................................................................................................... 92

References cited ................................................................................................................................................... 95

Appendix 1:.......................................................................................................................................................... 99

Appendix 2:.......................................................................................................................................................... 113
vi Contents

FIGURES

Page

1–3. Maps showing:

1. Physiography, hydrologic provinces, and physical features of the Central Arizona 

Basins study area .......................................................................................................................... 4

2. Principal streams by type of streamflow and generalized geology of the Central Arizona 

Basins study area .......................................................................................................................... 5

3. U.S. Geological Survey surface-water quality monitoring stations in the Central Arizona 

Basins study area and National Atmospheric Deposition Program precipitation-chemistry

stations in Arizona. ....................................................................................................................... 10

4–10. Graphs showing:

4. Temporal distribution of samples from surface-water quality monitoring stations in the 

Central Arizona Basins study area, through 1998 ........................................................................ 15

5. Discharge-duration curve and percentage of total samples collected by deciles of 

discharge at selected surface-water quality monitoring stations in the Central Arizona 

Basins study area, through 1998................................................................................................... 18

6. Distributions of stream properties and water-chemistry constituent concentrations at 

surface-water quality monitoring stations in the Central Arizona Basins study area, 

through 1998................................................................................................................................. 26

7. Distributions of precipitation-volume weighted annual average pH, dissolved solids, 

ammonia, nitrate, and total nitrogen concentrations in precipitation at three 

National Atmospheric Deposition Program monitoring stations in Arizona, 1980–96 ............... 39

8. Relation of stream properties and water-chemistry constituent concentrations to season 

and stream discharge, at selected surface-water quality monitoring stations in the 

Central Arizona Basins study area, October 1995 through September 1998 ............................... 40

9. Dissolved-solids concentrations at the Salt River below Stewart Mountain Dam and the 

annual combined inflow from the Salt River and Tonto Creek to the reservoirs upstream 

from this station, 1950–98 ............................................................................................................ 62

10. Seasonal and annual trends in logged monthly statistics of mean daily flow values at 

selected surface-water quality monitoring stations in the Central Arizona Basins

study area...................................................................................................................................... 73

11. Map showing annual stream loads of dissolved solids at selected surface-water quality 

monitoring stations in the Central Arizona Basins study area, water year 1997.................................. 85

12. Map showing annual stream loads of total nitrogen and total phosphorus at selected 

surface-water quality monitoring stations in the Central Arizona Basins study area, 

water year 1997 .................................................................................................................................... 86

13. Graphs showing mean daily discharge and monthly stream load of nitrate and ammonia plus 

organic nitrogen at the Salt River near Roosevelt and at the Hassayampa River, Central Arizona 

Basins study area, 1996–98 .................................................................................................................. 88

14. Graphs showing temporal trends of estimated annual inputs of nitrogen and phosphorus 

to Arizona from fertilizer and from bodily waste from cattle on feed, dairy cattle,

and humans........................................................................................................................................... 91
Contents vii

TABLES

Page

1. Selected basin and streamflow characteristics for surface-water quality and streamflow 

monitoring stations in the Central Arizona Basins study area, through 1998 ...................................... 11

2. Number of surface-water quality samples for which data are stored in the local database,

by stream property and water-chemistry constituent, through 1998 .................................................... 14

3. Number of samples and percentage of total samples by months at water-quality monitoring 

stations in the Central Arizona Basins study area, through 1998......................................................... 17

4. Number of samples that exceeded designated-use water-quality standards at surface-water

quality monitoring stations in the Central Arizona Basins study area, through 1998 23

5. Number of samples that exceeded stream-reach specific water-quality standards for total

nitrogen and total phosphorus concentrations at surface-water quality monitoring

stations in the Central Arizona Basins study area, through 1998......................................................... 25

6. Recommended upper limits of dissolved-solids concentrations for selected water uses ..................... 33

7. Correlation of stream properties and water-chemistry constituent concentrations with

streamflow at surface-water quality monitoring stations in the Central Arizona Basins

study area, through 1998 ...................................................................................................................... 37

8. Selected upstream conditions that could affect surface-water quality at selected monitoring

stations in the Central Arizona Basins study area ................................................................................ 64

9. Paired-station and multiple-station comparisons of stream properties and water-chemistry

constituent concentrations at selected surface-water quality monitoring stations in the Central

Arizona Basins study area 65

10. Monotonic trends in stream properties and water-chemistry constituent concentrations at

selected surface-water quality monitoring stations in the Central Arizona Basins

study area, through 1998 77

11. Annual streamflow for water years 1996–98 and mean annual streamflow for the period

of record at selected surface-water quality monitoring stations in the Central Arizona

Basins study area .................................................................................................................................. 79

12. Annual stream load and yield of selected water-chemistry constituents at selected 

surface-water quality monitoring stations in the Central Arizona Basins study area,

water years 1996–98............................................................................................................................. 80

13. Average annual input of dissolved solids from precipitation to drainage basins of selected

surface-water quality monitoring stations in the Central Arizona Basins study area........................... 83

14. Estimated annual input of nitrogen and phosphorus from various sources to drainage

basins of selected surface-water quality monitoring stations in the Central Arizona

Basins study area .................................................................................................................................. 89

15. Annual stream load of nutrients for 1996–98 as a percentage of the estimated annual

inputs for selected basins in the Central Arizona Basins study area .................................................... 92
viii Contents

CONVERSION FACTORS AND DATUMS

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8°C)+32

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29)—a geodetic datum 

derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level

Datum of 1929; horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27). Altitude, as 

used in this report, refers to distance above or below NGVD 29.

ABBREVIATED WATER-QUALITY UNITS

Chemical concentration and water temperature are given only in metric units. Chemical concentration in water is given in 

milligrams per liter (mg/L) or micrograms per liter (μg/L). Milligrams per liter is a unit expressing the solute mass 

(milligrams) per unit volume (liter) of water. One thousand micrograms per liter is equivalent to 1 milligram per liter. For 

concentrations lower than 7,000 milligrams per liter, the numerical value is about the same as for concentrations in parts per 

million. Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25°C). 

ABBREVIATIONS

NAWQA National Water-Quality Assessment

NWIS National Water Information System

SEP Standard error of prediction, as a percentage 

SMCL Secondary Maximum Contaminant Level

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

Multiply By To obtain

centimeter (cm) 0.3937 inch

meter (m) 3.281 foot 

kilometer (km) 0.6214 mile 

square kilometer (km2) 247.1 acre

square kilometer (km2) 0.3861 square mile

liter (L) 1.057 quart 

liter (L) 0.2642 gallon 

cubic meter per second (m3/s) 35.31 cubic foot per second 

cubic meter per second (m3/s) 70.07 acre-foot per day 

cubic meter per second (m3/s) 22.83 million gallons per day 

kilogram (kg) 2.205 pound avoirdupois

kilograms per square kilometer per year (kg/km2)/yr 0.008921 pounds per acre per year
 1

Assessment of Selected Inorganic Constituents in Streams in the 

Central Arizona Basins Study Area, Arizona and Northern Mexico, 

through 1998

By David W. Anning

Abstract 

Stream properties and water-chemistry constituent concentrations from data collected by the National 

Water-Quality Assessment and other U.S. Geological Survey water-quality programs were analyzed to 

(1) assess water quality, (2) determine natural and human factors affecting water quality, and (3) compute 

stream loads for the surface-water resources in the Central Arizona Basins study area. Stream 

temperature, pH, dissolved-oxygen concentration and percent saturation, and dissolved-solids, 

suspended-sediment, and nutrient concentration data collected at 41 stream-water quality monitoring 

stations through water year 1998 were used in this assessment. 

Water-quality standards applicable to the stream properties and water-chemistry constituent 

concentration data for the stations investigated in this study generally were met, although there were 

some exceedences. In a few samples from the White River, the Black River, and the Salt River below 

Stewart Mountain Dam, the pH in reaches designated as a domestic drinking water source was higher 

than the State of Arizona standard. More than half of the samples from the Salt River below Stewart 

Mountain Dam and almost all of the samples from the stations on the Central Arizona Project Canal—

two of the three most important surface-water sources used for drinking water in the Central Arizona 

Basins study area—exceeded the U.S. Environmental Protection Agency drinking water Secondary 

Maximum Contaminant Level for dissolved solids. Two reach-specific standards for nutrients established 

by the State of Arizona were exceeded many times: (1) the annual mean concentration of total 

phosphorus was exceeded during several years at stations on the main stems of the Salt and Verde Rivers, 

and (2) the annual mean concentration of total nitrogen was exceeded during several years at the Salt 

River near Roosevelt and at the Salt River below Stewart Mountain Dam. 

Stream properties and water-chemistry constituent concentrations were related to streamflow, season, 

water management, stream permanence, and land and water use. Dissolved-oxygen percent saturation, 

pH, and nutrient concentrations were dependent on stream regulation, stream permanence, and upstream 

disposal of wastewater. Seasonality and correlation with streamflow were dependant on stream 

regulation, stream permanence, and upstream disposal of wastewater. 

Temporal trends in streamflow, stream properties, and water-chemistry constituent concentrations 

were common in streams in the Central Arizona Basins study area. Temporal trends in the streamflow of 

unregulated perennial reaches in the Central Highlands tended to be higher from 1900 through the 1930s, 

lower from the 1940s through the 1970s, and high again after the 1970s. This is similar to the pattern 

observed for the mean annual precipitation for the Southwestern United States and indicates long-term 

trends in flow of streams draining the Central Highlands were driven by long-term trends in climate. 
2 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Streamflow increased over the period of record at 

stations on effluent-dependent reaches as a result 

of the increase in the urban population and 

associated wastewater returns to the Salt and 

Gila Rivers in the Phoenix metropolitan area and 

the Santa Cruz River in the Tucson metropolitan 

area. Concentrations of dissolved solids decreased 

in the Salt River below Stewart Mountain Dam and 

in the Verde River below Bartlett Dam. 

This decrease represents an improvement in the 

water quality and resulted from a concurrent 

increase in the amount of runoff entering the 

reservoirs. 

Stream loads of water-chemistry constituents 

were compared at different locations along the 

streams with one another, and stream loads were 

compared to upstream inputs of the constituent 

from natural and anthropogenic sources to 

determine the relative importance of different 

sources and to determine the fate of the waterchemistry constituent. Of the dissolved solids 

transported into the Basin and Range Lowlands 

each year from the Central Arizona Project Canal 

and from streams draining the Central Highlands, 

about 1.2 billion kilograms accumulated in the 

soil, unsaturated zone, and aquifers in agricultural 

and urban areas as a result of irrigating crops and 

urban vegetation. Stream loads of phosphorus 

decreased from the 91st Avenue WastewaterTreatment Plant downstream to the Gila River at 

Gillespie Dam, probably as a result of adsorption 

of phosphorus to the streambed sediments. In this 

same reach, stream loads of nitrogen increased, 

most likely because of inputs from fertilizers. 

The annual mass of nitrogen and phosphorus 

input to developed basins from quantifiable 

sources was much larger than the mass input to 

basins that had little or no municipal or agricultural development. These computed inputs exclude 

the mass of nitrogen and phosphorus from sources 

such as geologic formations and soils that could 

not be quantified. The quantifiable annual inputs 

of nitrogen and phosphorus for the upper 

Salt River Basin and the upper Verde River Basin 

were similar to those for the West Clear Creek 

Basin. This similarity suggests that the small 

amount of municipal and agricultural development 

in the upper Salt River and the upper Verde River 

Basins did not greatly change the basin input flux. 

For basins with minimal urban and agricultural 

development, the largest quantifiable source of 

nitrogen was precipitation, and the largest source 

of phosphorus was human bodily waste treated by 

sewer and septic systems. This was in contrast to 

developed basins, for which fertilizer was the 

largest quantifiable source of both nutrients. 

For most basins examined, quantifiable inputs of 

nitrogen and phosphorus from nonpoint sources 

were greater than inputs from point sources. 

This relation emphasizes the importance of 

land- and water-management policies that protect 

surface-water resources from nonpoint sources 

of nutrients as well as from point sources. 

The amount of nitrogen and phosphorus transported out of basins was a small fraction of the total 

for the quantifiable inputs. This result indicated 

that most of the nutrients input to basins were not 

transported out of the basins in surface water, but 

rather were transported to the subsurface (the soil, 

unsaturated zone, or aquifer), released to the 

atmosphere (such as volatilized ammonia), or 

incorporated into the biomass. 

INTRODUCTION

The objective of the National Water-Quality 

Assessment (NAWQA) program is to describe the 

status and trends in the quality of a large, representative 

part of the Nation’s surface-water and ground-water 

resources and to provide a sound, scientific understanding of the primary natural and human factors affecting 

the quality of these resources (Gilliom and others, 

1995). As part of the NAWQA program design, 

59 study areas were selected to represent most of the 

major river basins and aquifer systems in the United 

States. The Central Arizona Basins study area includes 

most of central and southern Arizona and a small area 

in northern Mexico. The study area is one of the most 

arid study areas of the NAWQA program and differs 

from most others because the demand for water 

exceeds the available renewable water supply. This 

imbalance has resulted in a managed hydrologic 

system and also places a high value on the quality of 
Introduction 3

water. The primary natural and human factors that 

affect the quality of surface-water and ground-water 

resources in this arid region will be described and 

evaluated as part of the Central Arizona Basins 

NAWQA program. 

As part of the NAWQA design, each study area 

conducts surface-water quality, ground-water quality, 

and aquatic-ecology investigations on a multiyear cycle 

beginning with 2 years of planning and evaluation of 

existing information, 3 years of intensive data 

collection and analysis, and 5 years of low-intensity 

data collection. In the Central Arizona Basins study 

area, the intensive data collection and analysis were 

conducted from 1996 to 1998 and included monthly 

sampling at nine surface-water quality stations and 

twice-per-month sampling at two surface-water quality 

stations. Field measurements were made for stream 

temperature, pH, dissolved oxygen, specific 

conductance, and alkalinity, and samples were 

analyzed for concentrations of major ions, dissolved 

solids, suspended sediment, nutrients, pesticides, and 

volatile organic compounds. Results from these 

measurements and analyses are used by the local 

NAWQA program to determine natural and human 

factors that affect the water quality in the study area. 

Data from these 11 stations, as well as data from 

stations in the other 58 study areas, are synthesized by 

the NAWQA program at the national level to determine 

the spatial and temporal trends in water quality and the 

natural and human factors that affect water quality 

across the country. 

Purpose and Scope 

This report presents results from an analysis of 

streamflow and water-quality conditions for surfacewater resources in the Central Arizona Basins study 

area collected by the NAWQA and other USGS waterquality monitoring programs. This report contains the 

results of three interpretive analyses of surface-water 

quality: (1) an assessment of the quality of surfacewater resources by a comparison of data to regulatory 

standards, (2) an exploration of the relation of selected 

stream properties and water-chemistry constituent concentrations to natural and human factors, and (3) an 

investigation of the relation between stream loads of 

nutrients and dissolved solids and upstream conditions. 

The information provided in this report will provide a 

general understanding of the effect of natural and 

human factors on the water quality of the surface-water 

resources of central Arizona. 

Description of the Central Arizona Basins

Study Area

The study area covers 90,000 km2 within Arizona 

and northern Mexico and includes six major river systems tributary to the Gila River: the San Pedro, Santa 

Cruz, Salt, Verde, Agua Fria, and Hassayampa Rivers 

(fig. 1). A brief description of the study area is given 

here; a more detailed discussion is provided by Cordy 

and others (1998).

Physiography

 The study area is within the Basin and Range 

Physiographic Province (Fenneman, 1931) and 

includes three hydrologic provinces: the Plateau 

Uplands, the Central Highlands, and the Basin and 

Range Lowlands (fig. 1; Arizona State Land Department, 1969). For simplicity, the small areas of the Plateau Uplands in the study area are considered in this 

report to be part of the Central Highlands Province.

 Basins of the Basin and Range Lowlands and the 

Central Highlands differ physiographically and hydrogeologically. In the Basin and Range Lowlands, the 

basins are greater in areal extent than the mountains 

and typically contain thick sequences of basin-fill 

deposits (fig. 2; Cordy and others, 1998, fig. 4; Robertson, 1991). In contrast, basins in the Central Highlands 

have a smaller areal extent than the mountains and contain shallow alluvial deposits. The Central Highlands is 

generally higher in altitude than the Basin and Range 

Lowlands, and streams that drain the Central Highlands 

flow into the Basin and Range Lowlands.

Climate

The climate of the study area ranges from arid to 

semiarid and is characterized by its spatial and annual 

variability (Cordy and others, 1998, figs. 6 and 7). 

Altitude is one of the most important factors 

controlling climate in the study area. Precipitation 

increases and temperature decreases with increasing 

altitude during all seasons of the year. The large 

topographic relief within the study area contributes to 

the spatial variability of precipitation and temperature.
4 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

113° 111°

35°

33°

0 20 40 60 80 100 MILES

0 20 40 60 80 100 KILOMETERS

BASIN AND RANGE

LOWLANDS 

 PROVINCE

CENTRAL HIGHLANDS PROVINCE

PLATEAU

UPLANDS

PROVINCE

Mogollon Rim

Color a

d

o

Riv e r

Gila

Salt

Gila

Cruz

San

River

Colorado

River

Verde

Riv e r

Santa

Pedro

Little

Colorado

River

Agua Fria

River

River

River

River

River

Bill William s R

Hassaya

mpa

River Canyon

MEXICO

NEW MEXICO

UTAH

NEVADA

CALIFORNIA

ARIZONA

Cananea

TUCSON

Tubac

Eloy

Kelvin

Globe

Miami

McNary

Pinetop-

 Lakeside

Roosevelt

Payson

Camp 

Verde

Sedona

Page

Clarkdale

Rock 

Springs

Chandler

Buckeye

Laveen

Gila Bend Winkelman

Safford

Nogales

PHOENIX

Prescott

Grand

Green

Valley

Rio Rico

Sierra

Vista

Saint

David

Stanfield

Cottonwood Cornville

Flagstaff

Springerville

 Dewey

Yuma

EXPLANATION

HYDROLOGIC PROVINCE BOUNDARY

Base from U.S. Geological Survey 

digital data, 1:250,000, 1990

CENTRAL ARIZONA BASINS STUDY AREA BOUNDARY

CENTRAL ARIZONA PROJECT CANAL

STREAMS

Gillespie Dam

Mormon

Flat Dam

Granite Reef

Dam

Horseshoe Dam

Bartlett

Dam

Stewart

Mountain

Dam

 

Parker Dam

Horse

Mesa 

Dam

Roosevelt 

Dam

New Waddell

Dam

Coolidge

Dam

AshurstHayden 

Dam

Figure 1. Physiography, hydrologic provinces, and physical features of the Central Arizona Basins study area.
Introduction 5

Basin fill

GEOLOGY

Consolidated rock

STREAMFLOW TYPE

HYDROLOGIC PROVINCE

BOUNDARY

Unregulated perennial

Unregulated ephemeral or

intermittent

Regulated

Effluent-dependent

EXPLANATION

DAM

Central

Highlands

Province

Basin and

Range

Lowlands

Province

Base from U.S. Geological Survey

digital data, 1:100,000, 1972

Albers Equal-Area Conic projection

Standard parallels 29°30', 45°30',

central meridian –111°30'

0

0

0 25 50 75 KILOMETERS

25 50 75 MILES

CENTRAL ARIZONA BASINS

STUDY AREA BOUNDARY

COUNTY BOUNDARY

35°

34°

33°

32°

113°

112°

111°

110°

PINAL

SANTA

CRUZ

PIMA

YAVAPAI

NAVAJO

APACHE

COCHISE

COCONINO

MARICOPA

GILA

ARIZONA

MEXICO

R

Tonto Cr

Santa

San

Salt

Lake

Pleasant Bartlett Res.

Wash

Beaver Cr

Dry Beaver Cr

CENTRAL

PROJECT

CANAL

Cruz

Altar Wash

Pedro

River

River

Ba bocomari R

Rillito Cr

Pantano W.

Cienega Cr

Brawley Wash

El Tiro Wash

Davidson W.

Aravaipa Creek

Apache

Lake

Syc amoreCr

Gila S

River anta Cruz Wash

River

Theodore

Roosevelt Lake

Pinto

Pinal

Quee n Creek

Waterman Wash

R

Verde

Sycamore Cre ek C

Bi g

Chino

Wash

Sedona

Cottonwood Cornville

Clarkdale

Granite Cr

BitterCr

Creek

Oak

Camp

Verde

Dewey Prescott

River Verde

Rock

Springs

WestClearCr

East

French

Gulch

River

Bonito

Canyon Creek

Creek

CoonCr

Roosevelt

C herry

Cibeque Cr Carri zo

Creek

River Whitee

Black

River

Cr

McNary

Pinetop-Lakeside

Globe

Kelvin

San

Carlos

Reservoir

C r Cr

M ni eralCr

Miami

Donnelly Wash

PHOENIX

Buckeye

Laveen

Hassayampa

River

Cave

Cr

Jackrabbit

Fria Agua

Wash

Chandler

Gila

Bend

Eloy Stanfield

Painted

Rock

Reservoir

(Intermittent)

ARIZONA

Wet Be aver Cr

Saint

David

Nogales

Payson

Flagstaff

Winkelman

Green Valley

Cananea

Sierra

Vista

Tubac

Rio

Rico

TUCSON

New

Santa Rosa

Tan gle Cr

Horseshoe Res.

GREENLEE

GRAHAM

Dri p

W

pi

a

n

s

g

h

Springs W

Figure 2. Principal streams by type of streamflow and generalized geology of the Central Arizona Basins study area

(geologic information modified from Reynolds, 1988; and Coordinacion General de Los Servicios Nationales de

Estadistica, Geografia E Informatica, 1981, 1983a, 1983b; type of streamflow information modified from Brown and others,

1981; and State of Arizona, 1996).
6 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Temperatures range from more than 46°C on summer 

afternoons in the lowest valleys to less than -18°C on 

winter nights in the higher mountains (Sellers and Hill, 

1974). Mean annual precipitation ranges from less than 

25 cm on the lowest valley floors to more than 64 cm at 

some places in the mountains of the Central Highlands 

(Sellers and Hill, 1974). Annual precipitation increases 

from northwest to southeast in the Basin and Range 

Lowlands and from southwest to northeast in the 

Central Highlands. Winter precipitation constitutes 

about 60 percent of annual precipitation throughout the 

Central Highlands (Arizona State Climatologist, 1975). 

In the Basin and Range Lowlands, winter precipitation 

constitutes about 35 percent of annual precipitation in 

the southeastern part, and about 60 percent in the 

northwestern part. Throughout the study area, the 

variability of annual precipitation is high; annual 

precipitation in wet years can be three times greater 

than in dry years.

Surface-Water Hydrology

The study area contains six major rivers tributary to 

the middle Gila River: the San Pedro River, the Santa 

Cruz River, the Salt River, the Verde River (tributary to 

the Salt River), the Agua Fria River, and the Hassayampa River (fig. 1). The upper reaches of the Salt, 

Verde, Agua Fria, and Hassayampa Rivers drain the 

Central Highlands, and the lower reaches of these rivers, as well as the San Pedro and the Santa Cruz Rivers, 

drain the Basin and Range Lowlands. The Gila River 

upstream from the study area boundary at Coolidge 

Dam drains areas in the eastern Central Highlands that 

extend into New Mexico. 

Mean annual streamflow in this arid to semiarid 

study area is low and parallels the spatial patterns of 

mean annual precipitation (Cordy and others, 1998, 

figs. 6 and 8). Mean annual streamflow for drainage 

basins in the Central Highlands is greater than that of 

drainage basins in the Basin and Range Lowlands. For 

1951–80, mean annual streamflow1 ranged from less 

than 0.3 cm in some areas of the Basin and Range Lowlands to more than 25 cm in higher altitudes of the Central Highlands (Gebert and others, 1987). 

1This annual streamflow, which is calculated as the runoff volume 

divided by the drainage area, is reported as an average depth over the entire 

basin.

The upper Agua Fria, Gila, Salt, and Verde Rivers 

have the greatest mean annual streamflow (Cordy 

and others, 1998, fig. 18) and provide the largest 

supplies of nonimported surface water in the study 

area. These streams and their tributaries drain the 

Central Highlands and flow into large reservoirs near 

the boundary with the Basin and Range Lowlands. 

The reservoirs on the Agua Fria, Gila, Salt, and Verde 

Rivers capture about 85 percent of the flow in these 

streams (Cordy and others, 1998) and maximize use of 

the water supply by allowing the surface water to be 

used throughout the year. Diversions from the 

reservoirs are delivered to a large area in the Basin and 

Range Lowlands through a large network of canals 

(Cordy and others, 1998, fig. 12). Where surface water 

is unavailable, ground water is used. Throughout much 

of the Basin and Range Lowlands in the study area, 

ground-water withdrawals have exceeded ground-water 

recharge, and as a result, ground-water levels have 

declined more than 20 m in many areas (Anderson and 

others, 1992, pl. 2). The Central Arizona Project was 

built to reduce ground-water overdraft by importing 

water from the Colorado River to central Arizona. The 

Central Arizona Project began providing water to users 

in 1985, and is the single largest source of surface 

water in the study area. In 1997, the annual flow in the 

Central Arizona Project Canal2 where it enters the 

study area was about 80 percent of the combined 

average annual outflows (for the period of record) from 

reservoirs on the Agua Fria, Gila, Salt, and Verde 

Rivers. 

As a result of water-management practices, some 

aspects of the hydrologic system have been significantly altered. More than 400 km of stream reaches

in the study area that were once perennial are now 

ephemeral because of stream diversions or lowered 

ground-water levels (Brown and others, 1981).

In contrast, more than 80 km of perennial reaches have 

been established or re-established by the discharge of 

treated municipal or irrigation wastewater back to

2The Central Arizona Project consists of a series of aqueducts and 

pumping plants that transport water from the Colorado River to central 

Arizona, including the Phoenix and Tucson metropolitan areas. Although 

the Bureau of Reclamation has established individual names for each 

aqueduct, the aqueducts will be referred to as the “Central Arizona Project 

Canal” in this report for simplicity and to be consistent with terminology 

used to name USGS water-quality monitoring stations on the aqueducts.
Stream-Property and Water-Chemistry Data 7

streams (Brown and others, 1981). These streams are 

termed “effluent dependent” in this report because their 

flow consists mostly of effluent, except during runoff 

events. Effluent-dependent streams are an important 

source of ground-water recharge and surface-water 

supply. In the study area, effluent is used for industrial 

cooling and for irrigation of turf and certain crops, such 

as cotton, that are not consumed by humans. 

The seasonal and spatial variability of streamflow 

in regulated and effluent-dependent reaches is controlled by water managers. Water typically is delivered 

from April through October for crop irrigation, and 

year-round for municipal use. Wastewater is discharged 

downstream from the users and follows the same seasonal patterns as the deliveries.

In contrast to streamflow in regulated and effluentdependent reaches, the seasonal and spatial variability 

of streamflow in unregulated reaches is determined by 

natural factors. Streamflow increases in response to 

runoff from frontal storms or snowmelt during the 

winter and spring, and from thunderstorms during the 

summer (Cordy and others, 1998, sites 1, 9, and 12, 

fig. 18). As a result, streamflow is seasonally bimodal 

in the study area. For streams in the Central Highlands, 

mean monthly streamflow typically is greater for 

winter months than for summer months; whereas, for 

streams in the Basin and Range Lowlands, mean 

monthly streamflow is equal to or greater for summer 

months than for winter months. During the spring and 

fall, streamflow is maintained by springs in the case of 

perennial streams, or may cease in the case of intermittent or ephemeral streams. 

The spatial distribution of perennial, intermittent, 

and ephemeral stream reaches is controlled by 

hydro-geologic factors. Stream reaches in areas with 

shallow bedrock and small and thin deposits of basin 

fill tend to be perennial, whereas stream reaches in 

areas with extensive and deep basin-fill deposits tend to 

be intermittent or ephemeral (fig. 2). As a result of this 

hydrogeologic control and the different typical basin 

structures for the two hydrologic provinces, most 

perennial streams in the study area occur in either the 

Central Highlands or in the higher altitude mountain 

ranges in the Basin and Range Lowlands, and most 

streams in the Basin and Range Lowlands are 

intermittent or ephemeral.

STREAM-PROPERTY AND WATER-CHEMISTRY 

DATA

Streamflow, stream-property, and water-chemistry 

data for surface-water stations in the study area that 

were collected as part of the NAWQA and other USGS 

surface-water quality monitoring programs were 

extracted from the National Water Information System 

(NWIS) database of the USGS and stored in a local 

database for analyses used in this study. These and 

other data in the NWIS database are available online 

(http://az.waterdata.usgs.gov/nwis) and from the 

Arizona District. Most of the data used in this report 

have been published in the annual series of reports, 

Water Resources Data, Arizona” (for example, 

Tadayon and others, 1999). 

 Sample results in the local database were evaluated to ensure that they were appropriate for at least 

one of the three interpretive analyses discussed in this 

report: (1) an assessment of water quality and comparison to regulatory standards, (2) an exploration of the 

relation of water quality to natural and human factors, 

and (3) an investigation of the relation between stream 

loads of dissolved solids and nutrients and upstream 

conditions. In addition to the water-quality data, daily 

mean values of streamflow measured by the USGS 

were used in the analyses. These data represent the 

continuous streamflow record and were used in several 

sections of the report to characterize the streamflow at 

the water-quality monitoring stations.

Data Selection and Station Categorization

Water-quality data for monitoring stations on 

streams, canals, and stormwater drainages were 

retrieved from the NWIS database for the local 

database. Only stations with 10 or more samples 

collected during a period greater than 12 months were 

included in the local database so that water-quality 

conditions would be adequately characterized at a 

given location. This eliminated several stations that 

were sampled only a few times. For cases in which 

replicate samples were collected, only the results from 

one sample were included in the local database. Timecomposited samples also were excluded so that the data 

set consisted only of results from instantaneous waterquality samples. The data retrieved from the NWIS 

database include stream properties (instantaneous 

discharge, stream temperature, pH, dissolved-oxygen 
8 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

concentration, dissolved-oxygen percent saturation) 

and concentrations of water-chemistry constituents 

(dissolved solids, suspended sediment, dissolved 

orthophosphate, total phosphorus, ammonia, nitrate, 

total ammonia plus organic nitrogen, and total 

nitrogen). Sample results were excluded from the local 

database if there were not data for at least one of the 

stream properties besides instantaneous streamflow or 

one of the water-chemistry constituents. 

The data in the local database were collected as a 

part of many different USGS water-quality programs 

with different objectives, and as a result, nitrogenconcentration data were recorded as several different 

species. Nitrogen-concentration data were aggregated 

into the following categories for this study so that 

comparisons between different stations and (or) time 

periods could be made: 

• Ammonia as N; includes ammonium ions and 

un-ionized ammonia.

• Nitrate as N; includes nitrate in every sample, 

and includes nitrite in many samples.

• Total ammonia plus organic nitrogen as N; 

includes ammonium ions, un-ionized ammonia, 

and nitrogen chemically bound in organic 

molecules such as proteins, amines, and amino 

acids.

• Total nitrogen as N; includes nitrate and total 

ammonia plus organic nitrogen (as described 

above). 

Data for samples analyzed for nitrate plus nitrite 

were considered nitrate data because in surface-water 

samples the nitrite concentration usually is

insignificant compared to the nitrate concentration. For 

nitrate and ammonia, laboratory analyses of filtered 

and unfiltered samples typically yield the same 

concentrations because these species occur 

predominantly in the dissolved phase. For this reason, 

concentration data presented in this report for ammonia 

and for nitrate include data from both filtered and 

unfiltered samples. This resulted in a more complete 

data set because there was a lack of consistency of 

filtered and unfiltered analyses between stations and 

over time at individual stations. 

A classification system for the stream reaches in 

the study area was devised to help determine some of 

the natural and human factors that affect surface-water 

quality. The system differentiates monitoring stations 

on the basis of stream regulation, streamflow 

permanence, streamflow seasonality, upstream disposal 

of municipal or agricultural wastewater, and land use. 

For the first level of the classification, water-quality 

monitoring stations are classified as being on 

predominantly unregulated, regulated, or effluentdependent stream reaches so that the effects of water 

storage, withdrawal and use of surface water, and 

return of wastewater to streams could be determined. 

Stations on unregulated reaches are further 

classified on the basis of stream permanence as either 

perennial, intermittent, or ephemeral, which contrasts 

the effects of climate and hydrogeology (Meinzer, 

1923, p. 57 and 58). Perennial streams have flow at all 

times that is sustained by surface flow from springs and 

(or) by ground water in reaches where the streambed is 

lower than the water table and is supplemented by 

rainfall or snowmelt runoff. Intermittent streams flow 

only during certain times of the year when continuous 

flow is sustained by surface flow from springs, ground 

water in reaches where the streambed is lower than the 

water table, or in some cases, by prolonged snowmelt. 

Ephemeral streams only flow in direct response to

precipitation and do not receive water from springs, 

ground water, or prolonged snowmelt.

Stations on unregulated reaches also were 

classified on the basis of the major land use in the 

respective basin. Land use of the basin upstream from 

the monitoring station was classified as minimally 

developed, urban, or agricultural on the basis of land 

uses identified in Cordy and others (1998, fig. 10). 

Stations on effluent-dependent reaches were 

further classified as receiving effluent from municipal 

wastewater-treatment plants, irrigation runoff, or both. 

Note that water-quality data for stations on effluentdependent reaches reflects the effects of water 

manage-ment and also of the land and water use 

associated with the two types of effluent. For example, 

effluent from a municipal wastewater-treatment plant 

represents municipal land and water uses, and effluent 

from irrigated fields represents agricultural land and 

water uses.

For many streams, physical properties and waterchemistry constituent concentrations are correlated 

with streamflow. For this reason, the seasonality of 

streamflow was classified to help define possible seasonal patterns in the stream properties and water-chemistry constituent concentrations. The seasonality of 

streamflow at stations was classified as uniform, unimodal, or bimodal on the basis of a 3-month running 

sum of the mean monthly streamflow volume. 
Stream-Property and Water-Chemistry Data 9

Streams with at least one 3-month running sum greater 

than 30 percent of the mean annual streamflow volume 

were considered seasonal; otherwise they were 

considered uniform. Streams with seasonal streamflow 

were further classified as unimodal or bimodal. 

Seasonally unimodal streams have one 3-month 

running sum of the mean monthly streamflow that 

contains more than 30 percent of the annual streamflow 

volume. Seasonally bimodal streams have two 3-month 

running sums that contain more than 30 percent of the 

annual streamflow volume and also are at least 

3 months apart. 

Distribution of Stream-Property and WaterChemistry Data

The local database contained results from 

5,980 samples collected at 41 water-quality monitoring 

stations (fig. 3, table 1) through water year 1998 that 

included results from more than 300 samples that were 

collected as part of the Central Arizona Basins 

NAWQA program during water years 1996–98. Not all 

samples were analyzed for every stream property and 

water-chemistry constituent. The percentage of samples for which data for a given property or constituent 

existed ranged from 32 percent for ammonia to 

87 percent for pH (table 2). Samples were not analyzed 

routinely for dissolved oxygen or nutrients until about 

1970; therefore, typically only streamflow, 

temperature, pH, dissolved-solids, and (or) suspendedsediment data are available for samples collected prior 

to about 1970.

Spatial distribution of stations

Data from 41 surface-water quality monitoring stations were included in the database—21 of these 

stations were within the Central Highlands, and 20 

were within the Basin and Range Lowlands (table 1). 

Twenty-seven stations were on unregulated stream 

reaches, 7 were on regulated reaches, and 7 were on 

effluent-dependent reaches (table 1). 

Of the 27 stations on unregulated reaches, 17 were 

on perennial stream reaches, 5 were on intermittent 

reaches, and 5 were on ephemeral reaches. Most of the 

stations on unregulated perennial reaches were in the 

Central Highlands, and the stations on ephemeral 

reaches were in the Basin and Range Lowlands. 

Although streamflow at two stations on the main stem 

of the upper Verde River and at three stations on 

Oak Creek was affected partly by diversions for 

agriculture during the summer, these stations were 

considered unregulated because streamflow was not 

impounded in a reservoir. The five stations on 

ephemeral reaches were on urban stormwater drainages 

rather than on natural streams. Because land use in 

these basins was mostly urban (Lopes and Fossum, 

1995), the water quality at these stations could have 

been affected more by land use than by the ephemeral 

nature of the streamflow. These stations were termed 

“unregulated urban ephemeral” reaches to recognize 

this strong bias in land use and were included to 

provide information on the urban stormwater, which is 

an important source of aquifer recharge in urban areas 

of the Basin and Range Lowlands. There were no

routinely sampled stations in the NWIS database that 

were on ephemeral reaches of minimally developed 

basins that could serve as a reference site for ambient 

water-quality conditions in ephemeral streams. 

Stations on regulated reaches tended to be near the 

boundary between the Basin and Range Lowlands and 

the Central Highlands (fig. 3). An exception is the Central Arizona Project Canal near Parker, Arizona, which 

is outside of the study area near the Arizona-California 

border. Lake Pleasant has been used to store water from 

the upper Agua Fria River since 1927, and water from 

the Central Arizona Project Canal since 1992. Inflow 

from the Central Arizona Project Canal typically has 

been much larger than inflow from the Agua Fria River. 

This difference is important to consider when interpreting the water-quality data for releases from the lake.3 

All the stations on effluent-dependent reaches are 

in the Basin and Range Lowlands. On the upper Santa 

Cruz River, the stations at Rio Rico and Tubac are 

downstream from the Nogales International 

Wastewater-Treatment Plant, and the station at Cortaro 

is downstream from two wastewater-treatment plants 

that serve the Tucson metropolitan area. Wastewater 

from much of metropolitan Phoenix is returned to the 

lower Salt River and is sampled at the 91st Avenue 

Wastewater-Treatment Plant outfall (site 38, fig. 3).

3Water-quality downstream from Lake Pleasant is monitored at the Agua 

Fria River below Waddell Dam and at the Central Arizona Project Canal 

near Phoenix. The New Waddell Dam, shown in fig.1, replaced Waddell 

Dam as part of the Central Arizona Project; however, for simplicity, the 

name of the water-quality monitoring station was not changed from “Agua 

Fria River below Waddell Dam.
10 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

0

CENTRAL ARIZONA BASINS

STUDY AREA BOUNDARY

PRECIPITATION-CHEMISTRY

STATION—From the National

Atmospheric Deposition program

network.

Effluent-dependent

Unregulated urban ephemeral

SURFACE-WATER QUALITY

MONITORING STATION -—

Number refers to table 1

EXPLANATION

Regulated

Unregulated intermittent

Unregulated perennial

INTERMITTENT OR

EPHEMERAL STREAM

PERENNIAL STREAM

32

GILA

PINAL

SANTA

CRUZ

NAVAJO

PIMA

YAVAPAI

APACHE

GREENLEE

GRAHAM

COCHISE

COCONINO

MARICOPA

GILA

Sedona

Cottonwood

Saint

David

Nogales

Gila

Bend

PHOENIX

Eloy

Miami Globe

Payson McNary

Flagstaff

Winkelman

Kelvin

Green Valley

Cananea

Sierra

Vista

Camp Verde

Clarkdale

Dewey

Rock

Springs

Pinetop-Lakeside

Stanfield

Buckeye

Chandler

Roosevelt

Laveen

TUCSON

Prescott

ARIZONA

MEXICO Yuma Colorado

River

River

Colorado River

Ri ver

Colorado Ri ver

Gila

Lit

Ri v er

Ri v er

Gila

Verde

San

Cruz

Santa

Pedro

River

ARIZO

M

NA

EXICO

A R I Z O N A

Tucson

Salt Rive r

tle

Phoenix

Flagstaff

CENTRAL ARIZONA

PROJECT CANAL

Tempe Mesa

Chandler

Laveen

Glendale

Sun City

Buckeye

Goodyear

Tolleson

Apache

Lake

C r Cr

Wash

Cr

Hassayampa River

New

R

Verde

River

West Clear Cr

Cave Cr

Syc amoreCr

Tonto Cr

C herry Creek

Cibeque Cr

Canyon Creek

Carri zo

Cr

River White

Black

Rive

B

r

onito

Gila

River

Santa

Cruz

Altar Wash

Waterman Wash Santa Rosa

San

Pedro

River

River

Santa Cruz Wash

Salt

River

Theodore

Roosevelt

Lake

Lake Pleasant

Painted

Rock

Reservoir

(Intermittent)

San

Carlos

Reservoir

East

Verde R

Pinto

Pinal CoonCr

Cr

Granite Cr

Sycamore Cre ek C

Bi g

Chino Wash

Wash

Ba bocomari R

Aravaipa Creek

Beaver Cr

Wet Be aver Cr

BitterCr

French

Gulch

Jackrabbit

Wash

M ni eralCr

Donnelly Wash

RillitoCr

Pantano W.

Cienega Cr

Tangle C r

Brawley Wash

El Tiro Wash

River

David son W.

Fria Agua

Quee n Creek

Oak

Creek

Dry

Beaver

Creek

Jackrabbit Wash

River

Gila

Salt

River

Hassayam

pa

Agua Fria River

River New

Skunk

Cr

Creek

Cave

River

River

Gila

0 10 MILES

CENTRAL ARIZONA

PROJECT CANAL

0 10 KILOMETERS

Indian

Bend

Wash

PHOENIX

Wash

CENTRAL

PROJECT

CANAL

ARIZONA

BUCKEYE

CANAL

SEE

INSET

ABOVE

60

10

85

10

60

89 17

40 39 38

32

23

Grand Canyon

Oliver

Knoll

Organ Pipe

30

1 2

41

18 21

20

19

16

15

14

8

9

7

11

10

37 36

35 34

33

22

35

25

30

31

29

28

17

13

12

4

22

20

10

26

27

24

3 6

5

Rio

Rico

Tubac

HYDROLOGIC PROVINCE

BOUNDARY

Central Highlands

Province

Basin and Range

Lowlands Province

COUNTY BOUNDARY

0 25 50 75 KILOMETERS

25 50 75 MILES

Dri pping Springs

Figure 3. U.S. Geological Survey surface-water quality monitoring stations in the Central Arizona Basins study area and National 

Atmospheric Deposition Program precipitation-chemistry stations in Arizona.
Stream-Property and Water-Chemistry Data 11

Table 1. Selected basin and streamflow characteristics for surface-water quality and streamflow monitoring stations in the 

Central Arizona Basins study area, through 1998

[Streamflow data are from the National Water Information System, except for streamflow at the 91st Avenue Wastewater Treatment Plant outfall, which is from the City of 

Phoenix. ---, not available or not applicable; n/c, not classified; land use is the predominant land use that affects the water quality at the station and may areally represent a 

small portion of the basin; seasonality is the type of seasonal distribution of streamflow and, if unimodal or bimodal, the month(s) in the middle of the mode; streamflow 

duration percentiles indicate the percentage of time that the tabulated discharge was equaled or exceeded] Site number

Station name 

(abbreviated)

U.S. Geological Surveystation identification number

Basin Characteristics

Streamflow 

type

Streamflow characteristics

Altitude, in metersDrainage area, insquare kilometersLand use 

Seasonality

Mean daily discharge, in

cubic meters per second

Mean

Duration percentile Period, in 

water 

Type Peak month 10 50 90 years

Central Highlands

1 Black River 09490500 1,324 3,190 Minimally 

developed

Unregulated 

perennial

Unimodal March 11.7 32.3 2.94 1.10 1957–98

2 White River 09494000 1,331 1,640 Minimally 

developed

Unregulated 

perennial

Unimodal April 13.8 15.6 2.49 .99 1957–98

3 Pinal Creek 09498400 853 505 n/c Unregulated 

perennial

Unimodal February .40 .34 .22 .14 1980–98

4 Salt River near 

Roosevelt1

09498500 664 11,200 Minimally 

developed

Unregulated 

perennial

Unimodal March 26.0 59.2 9.60 4.59 1914–98

5 Tonto Creek 09499000 769 1,750 Minimally 

developed

Unregulated 

perennial

Unimodal February 4.56 7.79 .65 .14 1941–98

6 Salt River below 

Stewart 

Mountain Dam

09502000 418 16,100 n/c Regulated Unimodal July 29.8 53.8 22.8 .08 1934–98

7 Verde River near 

Clarkdale

09504000 1,067 9,070 Minimally 

developed

Unregulated 

perennial

Unimodal February 5.18 5.47 2.35 2.07 1965–98

8 Verde River near 

Cornville

09504200 --- --- Agricultural Unregulated2

perennial

Unimodal February --- --- --- --- ---

9 Oak Creek near 

Sedona

09504420 1,271 603 Minimally 

developed

Unregulated 

perennial

Unimodal March3 2.60 4.47 .91 .76 1982–98

10 Oak Creek at Red 

Rock Crossing

09504440 --- 653 Minimally 

developed

Unregulated 

perennial

Unimodal March3 --- --- --- --- ---

11 Oak Creek near 

Cornville

09504500 1,058 919 Minimally 

developed

Unregulated 

perennial

Unimodal March 2.55 4.02 .91 .51 1940–45, 

1948–98

12 Verde River above 

West Clear 

Creek1

09505570 914 12,200 Agricultural Unregulated2

perennial

Unimodal February3 --- --- --- --- ---

13 West Clear Creek1 09505800 1,106 624 Minimally 

developed

Unregulated 

perennial

Unimodal March 1.91 3.17 .51 .42 1965–98

14 Verde River near 

Camp Verde

09506000 876 13,000 Agricultural Unregulated2

perennial

Unimodal February 12.9 22.8 5.32 2.27 1935–45, 

1988–98

15 East Verde River 09507980 762 857 Minimally 

developed

Unregulated 

perennial

Unimodal February 1.98 3.11 .68 .07 1962–64, 

1968–98

16 Wet Bottom Creek 09508300 707 94 Minimally 

developed

Unregulated 

intermittent

Unimodal February .44 .65 .01 0 1967–98

17 Verde River below 

Tangle Creek1

09508500 618 15,200 Minimally 

developed

Unregulated 

perennial

Unimodal February 16.7 26.6 6.74 3.45 1946–98

See footnotes at end of table.
12 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Table 1. Selected basin and streamflow characteristics for surface-water quality and streamflow monitoring stations in the Central Arizona Basins 

study area, through 1998—Continued Site number

Station name 

(abbreviated)

U.S. Geological Surveystation identification number

Basin Characteristics Streamflow characteristics

Altitude, in metersDrainage area, insquare kilometersLand use 

Streamflow 

type

Seasonality

Mean daily discharge,

in cubic meters per second

Mean

Duration percentile Period, 

in water 

Type Peak month 10 50 90 years

Central Highlands, continued

18 Verde River below 

Bartlett Dam4

09510000 479 16,000 n/c Regulated Unimodal February 16.8 33.7 8.49 1.22 1939–98

Unregulated 

perennial

March 24.4 44.2 8.07 3.79 1904–385

19 Turkey Creek 09512600 957 232 Minimally 

developed

Unregulated 

intermittent

Unimodal March .31 .57 0 0 1980–92

20 Agua Fria River 

near Rock 

Springs

09512800 549 2,880 Minimally 

developed

Unregulated 

intermittent

Unimodal February 2.66 3.34 .09 .01 1970–98

21 Agua Fria River 

below Waddell 

Dam

09513600 439 3,780 n/c Regulated Unimodal May3 --- --- --- --- ---

Basin and Range Lowlands

22 Central Arizona 

Project Canal 

near Parker

09426700 366 -- n/c Regulated Unimodal April 27.0 73.4 15.5 0 1985–98

23 Central Arizona 

Project Canal at 

Phoenix

09427100 351 -- n/c Regulated Unimodal July3 --- --- --- --- ---

24 Gila River at 

Winkelman

09470000 586 34,400 n/c Regulated Unimodal July 11.4 26.0 6.12 .27 1941–80, 

1984–94

25 San Pedro River at 

Charleston1

09471000 1,205 3,200 Agricultural Unregulated 

perennial

Unimodal August 1.33 1.64 .37 .10 1935–98

26 San Pedro River 

below Aravaipa 

Creek 

09473100 648 11,200 Agricultural Unregulated 

perennial

Unimodal August 1.36 1.39 .34 .15 1980–83

27 San Pedro River at 

Winkelman

09473500 587 11,500 Agricultural Unregulated 

intermittent

Unimodal August 1.25 1.50 .10 0 1966–78

28 Gila River at 

Kelvin1,4

09474000 532 46,600 n/c Regulated Bimodal January, August 13.5 28.2 8.04 .85 1928–98

Unregulated 

perennial

Unimodal July 22.4 48.1 5.83 .34 1911–27

29 Santa Cruz River at 

Rio Rico

09481710 0 2,600 n/c EffluentdependentBimodal January, 

August3

--- --- --- --- ---

30 Santa Cruz River at 

Tubac1

09481740 969 3,130 n/c EffluentdependentBimodal January, August .63 1.05 .45 .10 1996–98

See footnotes at end of table.
Stream-Property and Water-Chemistry Data 13

Basin and Range Lowlands, continued

31 Santa Cruz River at 

Cortaro1

09486500 650 9,070 n/c EffluentdependentBimodal August, January 1.53 1.87 .06 0 1940–47, 

1950–84, 

1990–98

32 Santa Cruz River 

near Laveen

09489000 311 22,200 n/c Unregulated 

intermittent

Unimodal January .58 .24 0 0 1940–98

33 Indian Bend Wash 

at Curry Road

09512162 351 212 Urban Unregulated 

urban 

ephemeral

Bimodal January, 

September

.19 0 0 0 1993–98

34 Box Culvert at 

48th Street 

Drain6

09512184 344 0.16 Urban Unregulated 

urban 

ephemeral

Bimodal January, 

September3

--- --- --- --- ---

35 27th Avenue at Salt 

River6

09512403 317 0.18 Urban Unregulated 

urban 

ephemeral

Bimodal January, 

September3

--- --- --- --- ---

36 43rd Avenue and 

Peoria Avenue6

09513885 373 0.01 Urban Unregulated 

urban 

ephemeral

Bimodal January, 

September3

--- --- --- --- ---

37 Olive Avenue and 

67th Avenue6

09513925 352 0.07 Urban Unregulated 

urban 

ephemeral

Bimodal January, 

September3

--- --- --- --- ---

38 91st Avenue 

WastewaterTreatment Plant 

outfall1

09512407 293 --- n/c EffluentdependentUniform --- 4.04 5.27 3.91 3.17 1996–98

39 Gila River at 

Buckeye 

Canal1,7

09514000 278 --- n/c EffluentdependentUniform --- 5.52 6.60 5.35 4.36 1997–98

40 Hassayampa 

River1

09517000 254 3,810 n/c EffluentdependentUnimodal January 1.80 3.11 1.44 .42 1990–98

41 Gila River at 

Gillespie Dam

09518000 230 128,000 n/c EffluentdependentUnimodal February 15.4 9.68 2.10 .34 1940–71, 

1974–98

1Station is part of the National Water-Quality Assessment program surface-water-quality network.

2Low flow is regulated partly by diversions.

3Seasonality estimated on the basis of data from nearby upstream or downstream stations, or on the basis of monthly streamflow data. 4Streamflow type and characteristics are given for period before and after streamflow was regulated by an upstream dam.

5Record is fragmented into several periods.

6Storm runoff in an unnatural channel is monitored at this station. 7Includes samples from U.S. Geological Survey station 09513990, Gila River above diversions at the head of Buckeye Canal.

Table 1. Selected basin and streamflow characteristics for surface-water quality and streamflow monitoring stations in the Central Arizona Basins 

study area, through 1998—Continued Site number

Station name 

(abbreviated)

U.S. Geological Surveystation identification number

Basin Characteristics Streamflow characteristics

Altitude, in metersDrainage area, insquare kilometersLand use 

Streamflow 

type

Seasonality

Mean daily discharge,

in cubic meters per second

Mean

Duration percentile Period, 

in water 

Type Peak month 10 50 90 years
14 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

This effluent flows into the Gila River and is diverted 

almost entirely into the Buckeye Canal downstream 

from the mouth of the Agua Fria River. Water quality 

of the Gila River at Buckeye Canal was characterized 

by samples collected from two stations: USGS station 

09513990, which is at the canal intake, and USGS 

station 09514000, which was on the canal about a half 

a kilometer downstream from the intake. The initial 

samples collected for the Central Arizona Basins 

NAWQA program were collected at 09513990; 

however, after 6 months, samples were collected 

downstream at 09514000 because sampling conditions 

were better. In this report, data for these stations were 

combined under the station name “Gila River at 

Buckeye Canal” (site 39, fig. 3) to simplify data 

analysis and reporting. Water in the Buckeye Canal is 

used for irrigation and is supplemented with pumped 

ground water and irrigation return flows along the 

length of the canal. At the terminus, the Buckeye Canal 

empties into the otherwise ephemeral Hassayampa 

River. The Hassayampa River (site 40, fig. 3) is 

sampled downstream from the Buckeye Canal outfall 

and flows into the Gila River, which is sampled at 

Gillespie Dam (site 41, fig. 3) near the downstream end 

of the study area. Except during periods of runoff, flow 

in the Buckeye Canal, the Hassayampa River, and the 

Gila River at Gillespie Dam consists of a mix of 

municipal and irrigation wastewater.

Temporal Distribution of Water-Quality Samples

Natural and human factors that affect water quality 

change seasonally and annually, and consequently, 

water quality also changes seasonally and annually. 

Therefore, samples from each season are needed to 

characterize the seasonal factors affecting water 

quality. Water-quality data from stations at which 

samples are collected only during certain seasons are 

biased by the predominant factors that affect water 

quality during those seasons. The temporal distribution 

of samples must be considered when comparing waterquality data from different stations to ensure that 

differences in the data are the result of differences in 

the natural and human factors affecting the water 

quality at the stations rather than the result of 

differences in sample collection dates.

The temporal distribution of sample collection 

dates varies greatly among stations (fig. 4). From the 

early 1950s to the mid-1970s, most samples were 

collected downstream from the major surface-water 

reservoirs at a high frequency and typically were 

analyzed for stream temperature, pH, suspended 

sediment, dissolved solids, and specific conductance. 

Beginning in the 1970s, samples were collected from a 

larger number of stations and typically included the 

same stream properties and water-chemistry constituent concentrations previously listed, as well as nutrients; however, the frequency was typically much lower. 

Only a few stations have similar periods of record and 

sampling frequencies. Stations typically were sampled 

using one of five different sampling frequencies: 

quarterly (such as the Central Arizona Project Canal 

near Parker), every other month (such as the East 

Verde River), monthly (such as the Agua Fria River 

near Rock Springs), or twice per month (such as the 

Hassayampa River during 1997). Sampling was storm 

based for stations on unregulated urban ephemeral 

reaches, such as Indian Bend Wash at Curry Road. 

At many stations, the sampling frequency changed 

during the period of record.

Table 2. Number of surface-water quality samples for which 

data are stored in the local database, by stream property and 

water-chemistry constituent, through 1998 

[Note: There is a total of 5,980 samples collected from 41 water-quality 

monitoring stations in the local database]

Number of samples for which 

data are stored 

Stream property or waterchemistry constituent NumberAs a percentage 

of all samples

Instantaneous discharge 3,523 59

Stream temperature 3,791 63

pH 5,302 87

Dissolved oxygen:

Concentration

Percent saturation 

2,802

1,974

47

33

Suspended sediment 2,900 48

Dissolved solids 3,951 66

Phosphorus:

Total

Dissolved, ortho

3,086

2,029

52

34

Nitrogen:

Total

Nitrate

Ammonia

Total ammonia plus organic

2,996

3,270

1,931

3,008

50

55

32

50
Stream-Property and Water-Chemistry Data 15

Gila River at Gillespie Dam

Hassayampa River

Gila River at Buckeye Canal

91st Avenue Wastewater-Treatment Plant

Olive Avenue and 67th Avenue

43rd Avenue and Peoria Avenue

27th Avenue at Salt River

Box Culvert at 48th Street Drain

Indian Bend Wash at Curry Road

Santa Cruz River at Laveen

Santa Cruz River at Cortaro

Santa Cruz River at Tubac

Santa Cruz River at Rio Rico

Gila River at Kelvin

San Pedro River at Winkelman

San Pedro River below Aravaipa Creek

San Pedro River at Charleston

Gila River at Winkelman

Central Arizona Project Canal at Phoenix

Central Arizona Project Canal near Parker

Agua Fria River below Waddell Dam

Agua Fria River near Rock Springs

Turkey Creek

Verde River below Bartlett Dam

Verde River below Tangle Creek

Wet Bottom Creek

East Verde River

Verde River near Camp Verde

West Clear Creek

Verde River above West Clear Creek

Oak Creek near Cornville

Oak Creek at Red Rock Crossing

Oak Creek near Sedona

Verde River near Cornville

Verde River near Clarkdale

Salt River below Stewart Mountain Dam

Tonto Creek

Salt River near Roosevelt

Pinal Creek

White River

Black River

SURFACE-WATER QUALITY MONITORING STATION

1950 2000

CENTRAL HIGHLANDS

1955 1960 1965 1970 1975 1980 1985 1990 1995

SURFACE-WATER QUALITY MONITORING STATION

1950 2000

BASIN AND RANGE LOWLANDS

1955 1960 1965 1970 1975 1980 1985 1990 1995

Effluent-dependent

SURFACE-WATER QUALITY MONITORING STATION BY TYPE OF REACH ONE COLLECTED SAMPLE

EXPLANATION

Unregulated intermittent

Unregulated perennial Unregulated urban ephemeral

Regulated

Figure 4. Temporal distribution of samples from surface-water quality monitoring stations in the Central Arizona Basins 

study area, through 1998.
16 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Generally, samples were distributed uniformly 

throughout the seasons (table 3). A perfectly uniform 

seasonal distribution has 17 percent of the samples 

collected in each 2-month period, as tabulated in

table 3. The standard deviation of the percent of 

samples for the 2-month periods provides a measure of 

the uniformity of the seasonal sample distribution; a 

low standard deviation indicates a fairly uniform seasonal distribution. Samples from stations on unregulated perennial reaches tend to be distributed uniformly 

throughout the seasons, whereas samples from stations 

on unregulated urban ephemeral reaches tend to have 

bimodal distributions as a result of having no streamflow during some times of the year. Samples from the 

Agua Fria River below Waddell Dam are biased toward 

summer months because reservoir releases are greatest 

during this period owing to irrigation needs. 

Streamflow at the Time of Sampling

Stream properties and water-chemistry constituent 

concentrations can vary with streamflow because the 

relative contribution of constituents from different 

sources varies with streamflow, and because the 

activity of physical or biological processes that affect 

stream properties or water-chemistry constituent 

concentrations varies with streamflow. The distribution 

of samples with respect to streamflow is, therefore, 

important to consider when interpreting stream waterquality data. A bias in the values of stream properties 

and water-chemistry constituent concentrations may 

result from a bias in the distribution of samples with 

respect to streamflow at the time of sampling. Stations 

with a uniform distribution of samples with respect to 

streamflow at the time of sampling better represent 

water-quality conditions for the range of streamflow at 

that station. 

Graphs of streamflow duration (fig. 5) were 

constructed for all stations with continuous streamflow 

data by using daily values from the periods of record 

that are listed in table 3. The streamflow-duration curve 

shows the percentage of time during the period of 

streamflow record (table 1) that streamflow was equal 

to or less than a particular discharge. The percentage of 

total samples collected between two consecutive 

deciles4 of streamflow was superimposed on the 

streamflow-duration graphs. A station with a perfectly 

uniform distribution of samples with respect to 

streamflow has 10 percent of its samples between each 

decile of streamflow. 

The shape of the streamflow-duration curve 

provides information on the variability and sources of 

streamflow at each station. For stations on unregulated 

reaches, such as the Verde River near Clarkdale, the left 

side of the streamflow-duration curve with relatively 

mild slopes represents the percentage of time that base 

flow is maintained by steady ground-water inflow from 

springs or through the channel bottom. The right side 

of the streamflow-duration curve with relatively steep 

slopes represents the percentage of time that storm 

runoff and snowmelt are the main sources of 

streamflow. Stations on reaches with intermittent or 

ephemeral streamflow, such as the San Pedro River at 

Winkelman and Indian Bend Wash, respectively, have 

zero discharge for one or more percentiles of 

streamflow. For effluent-dependent reaches, the right 

side of the streamflow-duration curve with a steep 

slope represents the percentage of time that flow is 

primarily from storm runoff. This pattern is similar to 

that for stations on unregulated reaches. The middle 

and left side of the streamflow-duration curves, 

however, represent periods that streamflow is 

maintained mostly by municipal or agricultural 

wastewater rather than by spring flow. 

In general, the distribution of samples with respect 

to discharge was least biased for stations on unregulated perennial reaches. The samples from the San Pedro 

River at Charleston, the Black River, and the Verde 

River near Clarkdale were biased toward small discharges. The distribution of samples from West Clear 

Creek was bimodal with many samples that were 

collected at small or large discharges but few samples 

collected at medium discharges. The distribution of 

samples from stations on unregulated intermittent 

reaches and unregulated urban ephemeral reaches 

generally was biased with samples collected at larger 

discharges. In some cases this was simply because 

there is zero discharge for the lower streamflow 

percentiles (the stream is dry), such as at Wet Bottom 

Creek. For other cases, such as Turkey Creek, however, 

the percentage of total samples increased with increasing streamflow deciles. The distribution of samples 

from Indian Bend Wash (fig. 5) is representative of 

stations on unregulated urban ephemeral reaches that 

could be sampled only during periods of runoff. 

4A percentile is any one of the numbers or values in a series dividing the 

distribution of individuals in the series into 100 groups of equal frequency. 

Similarly, a decile is any one of the numbers or values in a series dividing 

the distribution of individuals in the series into 10 groups of equal 

frequency. 
Stream-Property and Water-Chemistry Data 17

Table 3. Number of samples and percentage of total samples by months at water-quality monitoring stations in the Central Arizona Basins

study area, through 1998

Station

Total 

Samples

Percentage of total samples 

Jan–Feb Mar–Apr May–June July–Aug Sept–Oct Nov–Dec

Standard 

deviation

Central Highlands

Black River 44 16 18 18 18 16 14 1.9

White River 45 18 16 20 18 16 13 2.3

Pinal Creek 161 16 17 17 17 16 17 .9

Salt River near Roosevelt 267 17 19 17 17 16 15 1.1

Tonto Creek 56 16 18 18 20 16 13 2.5

Salt River below Stewart Mountain Dam 509 14 18 19 20 18 11 3.3

Verde River near Clarkdale 192 15 18 18 18 18 14 1.9

Verde River near Cornville 24 21 17 17 17 13 17 2.6

Oak Creek near Sedona 23 17 13 17 17 17 17 1.8

Oak Creek at Red Rock Crossing 147 17 16 16 18 16 17 .7

Oak Creek near Cornville 35 11 26 17 14 20 12 5.5

Verde River above West Clear Creek 21 14 19 19 19 19 10 3.9

West Clear Creek 47 13 32 17 13 17 9 8.1

Verde River near Camp Verde 112 16 15 17 17 20 15 1.7

East Verde River 47 17 17 19 19 13 15 2.5

Wet Bottom Creek 182 21 18 14 12 13 23 4.4

Verde River below Tangle Creek 199 16 20 16 16 17 17 1.5

Verde River below Bartlett Dam 638 17 17 15 18 16 16 1.0

Turkey Creek 72 15 11 1 63 1 8 23.1

Agua Fria River near Rock Springs 168 16 16 17 18 17 15 1.1

Agua Fria River below Waddell Dam 75 7 32 28 27 7 0 13.7

Total 3,064

Basin and Range Lowlands 

Central Arizona Project near Parker 41 17 20 12 27 7 17 6.6

Central Arizona Project at Phoenix 123 15 18 15 18 17 17 1.3

Gila River at Winkelman 86 17 20 17 17 13 15 2.4

San Pedro River at Charleston 177 10 9 12 40 18 11 11.6

San Pedro River below Aravaipa Creek 134 16 17 17 17 16 17 .8

San Pedro River at Winkelman 120 13 9 6 46 16 10 14.7

Gila River at Kelvin 1,268 19 12 10 24 19 17 5.1

Santa Cruz River at Rio Rico 29 17 14 21 17 14 17 2.6

Santa Cruz River at Tubac 28 14 14 18 22 14 18 3.0

Santa Cruz River at Cortaro 15 13 13 13 20 27 13 5.6

Santa Cruz River at Laveen 16 31 25 0 6 31 6 14.1

Indian Bend Wash at Curry Road 19 37 5 0 26 16 16 13.5

Box Culvert at 48th Street Drain 26 15 15 8 31 12 19 8.0

27th Avenue at Salt River 28 25 18 0 29 4 25 12.1

43rd Avenue and Peoria Avenue 25 28 8 0 36 8 20 13.7

Olive Avenue and 67th Avenue 19 26 11 0 32 16 16 11.3

91st Avenue Wastewater-Treatment Plant 

outfall

24 17 17 17 17 13 21 2.7

Gila River at Buckeye Canal 40 23 15 15 15 13 20 3.8

Hassayampa River 50 20 14 12 24 18 12 4.8

Gila River at Gillespie Dam 648 14 12 15 25 19 14 4.6

Total 2,916
18 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

0.01

50

0.1

1

10

0.01

1,000

0.1

1

10

100

0.01

500

0.1

1

10

100

50

0

10

20

30

40

0.01

100

0.1

1

10

50

0

10

20

30

40

1

5,000

10

100

1,000

50

0

10

20

30

40

0.01

1,000

0.1

1

10

100

0.01

5,000

0.1

1

10

100

1,000

0.1

1,000

1

10

0 50 100 0

0

50 100

100

0

50

10

20

30

40

BLACK RIVER WHITE RIVER

TONTO CREEK

PINAL CREEK

0 50 100 0 50 100

0 50 100 0 50 100

0 50 100

0 50 100

CENTRAL HIGHLANDS

TURKEY CREEK

50

0

10

20

30

40

50

0

10

20

30

40

50

0

10

20

30

40

50

0

10

20

30

40

DISCHARGE, IN CUBIC METERS PER SECOND

PERCENTAGE OF TOTAL SAMPLES

PERCENTILE OF DISCHARGE

PERCENTAGE OF TOTAL WATERQUALITY SAMPLES BETWEEN

EACH DECILE OF DISCHARGE

DURATION

DISCHARGE–In cubic meters per

second, which streamflow was

equal to or less than for the

indicated percentage of time

Effluent-dependent

Unregulated urban ephemeral

SURFACE-WATER QUALITY

MONITORING STATION—

Color-coded by type of reach

EXPLANATION

Regulated

Unregulated intermittent

Unregulated perennial

SALT RIVER BELOW

STEWART MTN DAM

AGUA FRIA RIVER

NEAR ROCK SPRINGS

SALT RIVER

NEAR ROOSEVELT

Figure 5. Discharge-duration curve and percentage of total samples collected by deciles of discharge at selected surfacewater 

quality monitoring stations in the Central Arizona Basins study area, through 1998. Discharge percentiles are for the period of

record listed in table 1.
Stream-Property and Water-Chemistry Data 19

1

1,000

10

100

0

50

10

20

30

40

0.5

500

1

10

100

0

50

10

20

30

40

0.1

500

1

10

100

50

0

10

20

30

40

0.1

500

1

10

100

0

50

10

20

30

40

1

5,000

10

100

1,000

50

0

10

20

30

40

0.01

500

0.1

1

10

100

0

50

10

20

30

40

0.0001

100

0.001

0.01

0.1

1

10

0

50

10

20

30

40

1

5,000

10

100

1,000

50

0

10

20

30

40

0.1

5,000

1

10

100

1,000

50

0

10

20

30

40

WEST CLEAR CREEK EAST VERDE RIVER

CENTRAL HIGHLANDS

DISCHARGE, IN CUBIC METERS PER SECOND

PERCENTAGE OF TOTAL SAMPLES

PERCENTILE OF DISCHARGE

VERDE RIVER

NEAR CLARKDALE

OAK CREEK

NEAR SEDONA

VERDE RIVER

NEAR CAMP VERDE

VERDE RIVER

BELOW TANGLE CREEK

VERDE RIVER

BELOW BARTLETT DAM

OAK CREEK

NEAR CORNVILLE

0 50 100

0 50 100

0 50 100

0 50 100

0 50 100

0 50 100

0 50 100 0 50 100

WET BOTTOM CREEK

0 50 100

Figure 5. Continued.
20 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

0.05

500

0.1

1

10

100

50

0

10

20

30

40

0.01

1,000

0.1

1

10

100

50

0

10

20

30

40

0.01

1,000

0.1

1

10

100

50

0

10

20

30

40

0.05

100

0.1

1

10

50

0

10

20

30

40

0.01

500

0.1

1

10

100

50

0

10

20

30

40

0.01

5,000

0.1

1

10

100

1,000

50

0

10

20

30

40

0.01

10

0.1

1

50

0

10

20

30

40

0.01

1,000

0.1

1

10

100

50

0

10

20

30

40

100

0

20

40

60

80

GILA RIVER AT KELVIN

BASIN AND RANGE LOWLANDS

DISCHARGE, IN CUBIC METERS PER SECOND

PERCENTAGE OF TOTAL SAMPLES

PERCENTILE OF DISCHARGE

CAP* CANAL

NEAR PARKER

SAN PEDRO RIVER

BELOW ARAVAIPA CREEK

SANTA CRUZ RIVER

AT TUBAC

SANTA CRUZ RIVER

AT CORTARO

SANTA CRUZ RIVER

AT LAVEEN

SAN PEDRO RIVER

AT WINKELMAN

SAN PEDRO RIVER

AT CHARLESTON

GILA RIVER

AT WINKELMAN

0 50 100 0 50 100 0 50 100

0 50 100

0 50 100

0 50 100

0 50 100

0 50 100

0 50 100

0.01

1,000

0.1

1

10

100

Figure 5. Continued. *Central Arizona Project.
Stream-Property and Water-Chemistry Data 21

The distributions of samples from stations on regulated reaches were slightly biased with more samples 

that were collected at larger discharges; the Central 

Arizona Project Canal near Parker is an exception. 

Several samples were collected from this station 

shortly after the pumping plant was shut off, resulting 

in little or no flow at the time of sampling (represented 

by the bar to the left of the streamflow duration curve 

in figure 5). 

Of the stations on effluent-dependent reaches, the 

distributions of samples from the Santa Cruz River at 

Tubac and the Hassayampa River were unbiased with 

respect to discharge. The distributions of samples from 

the Santa Cruz River at Cortaro and the Gila River at 

Gillespie Dam were biased toward samples collected at 

larger discharges. The distribution for the 91st Avenue 

Wastewater-Treatment Plant outfall was biased toward 

samples collected at small discharges, in part because 

most samples were collected in the midmorning. 

At this station, discharge typically is small during the 

morning, increases during the day, and decreases 

during the night. This large diel variation in discharge 

does not occur at the stations on unregulated and 

regulated reaches; and therefore, biases in sampling 

time do not result in biases related to discharge at the 

time of sampling at stations on these reaches. 

Figure 5. Continued. *Wastewater-Treatment Plant

0.01

100

0.1

1

10

50

0

10

20

30

40

0.01

50

0.1

1

10

100

0

20

40

60

80

1

10

2

5

50

0

10

20

30

40

1

10

2

5

50

0

10

20

30

40

INDIAN BEND WASH

HASSAYAMPA RIVER

BASIN AND RANGE LOWLANDS

PERCENTILE OF DISCHARGE

DISCHARGE, IN CUBIC METERS PER SECOND

PERCENTAGE OF TOTAL SAMPLES

0 50 100

0.01

5,000

0.1

1

10

100

1,000

50

0

10

20

30

40

0 50 100

0 50 100

91st AVENUE WWTP*

OUTFALL

GILA RIVER

AT BUCKEYE CANAL

GILA RIVER

AT GILLESPIE DAM

0 50 100

0 50 100
22 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

ASSESSMENT OF WATER QUALITY BY 

COMPARISONS TO REGULATORY STANDARDS

Distributions of stream property and waterchemistry data at surface-water quality monitoring 

stations were examined and compared to regulatory 

standards to assess the suitability of surface water in 

the study area for specific uses. As part of the waterquality regulations (State of Arizona, 1996), specific 

designated uses have been assigned to the reaches of 

major streams in Arizona. The designated uses are: 

domestic water source; full and partial body contact; 

fish consumption; aquatic and wildlife, which is

subset into cold water fishery, warm water fishery, 

ephemeral stream, and effluent-dependent water; 

agricultural irrigation; and agricultural livestock 

watering. Water-quality standards for each stream 

property and water-chemistry constituent vary by 

designated use. In addition to the standards for 

designated uses, some stream reaches have specific 

water-quality standards for concentrations of total 

nitrogen and total phosphorous (State of Arizona, 

1996). Stream-property and water-chemistry data also 

were compared to U.S. Environmental Protection 

Agency (USEPA) Primary and Secondary DrinkingWater Regulations. These regulations apply to water 

supplied to the public; therefore, comparison of data to 

these standards indicates only the potential for actual 

exceedences of these regulations. Results from this 

assessment are limited in scope to only those stream 

properties and water-chemistry constituents examined; 

the suitability of the water for various uses may be 

limited by other concentrations of water-chemistry 

constituents not examined in this report. 

The assessment of the general water quality of 

streams in the study area was made by comparing data 

from the local database to the water-quality standards 

for designated uses and specific stream reaches at 36 of 

the 41 stations. Data from the five stations on unregulated urban ephemeral reaches were excluded because 

there are no applicable standards for ephemeral 

streams. Results of the comparison are presented for 

each station as the relative frequency of samples that 

exceeded designated-use water-quality standards for 

pH, dissolved oxygen, dissolved solids, ammonia, and 

nitrate (table 4), and stream-reach specific waterquality standards for total nitrogen and total phosphorus (table 5). This assessment is meant to reflect 

general conditions over several years and may not 

represent recent conditions; in fact, many of the

samples used in this assessment were collected before 

the standards were established. Complementing the 

comparison to water-quality standards is a graphical 

summary of the statistical distributions of stream 

temperature, pH, dissolved oxygen, dissolved solids, 

suspended sediment, and nutrients for each of the 

41 stations (fig. 6). Stations were grouped by hydrologic province and presented in downstream order, with 

the exception of stations on unregulated urban ephemeral reaches in the Basin and Range Lowlands. Plots 

for these stations were placed next to each other to 

facilitate visual comparison of the data. In addition, the 

statistical summaries of the stream properties and 

water-chemistry constituents in figure 6 are tabulated in 

appendix 1. 

Stream temperature, pH, and dissolved oxygen

Water temperature influences many physical, 

chemical, and biological characteristics of aquatic 

systems. Most chemical equilibria are dependent on 

physical properties of the water, such as temperature. 

Aquatic species have certain tolerances of exposure to 

high and low stream temperatures. Stream temperatures within the study area ranged from just above 0°C 

at several stations in the Central Highlands to 39°C at 

the San Pedro River at Winkelman, and are variable at 

any given station (fig. 6 and appendix 1). There are 

water-quality standards for temperature for surfacewater resources in the study area; however, they are 

generally specific to individual point sources. 

For example, in streams designated for cold water 

fishery uses, the water temperature should not increase 

more than 1°C because of inflows from point sources. 

Locations of surface-water quality monitoring stations 

included in this study were not appropriate for 

evaluating conformity to these types of standards.

Like water temperature, pH also influences many 

physical, chemical, and biological characteristics of 

aquatic systems. Most chemical equilibria are 

dependent on pH as well as temperature. Aquatic 

species have certain tolerances of compounds, such as 

un-ionized ammonia, that are pH and temperature 

dependent. Most of the samples from stations in the 

study area were alkaline (pH > 7 standard units), with 

the exception of some samples from three stations on 

unregulated urban ephemeral reaches—Box Culvert at 

48th Street drain, 43rd Avenue and Peoria Avenue, and 

Olive Street and 67th Avenue—that had values of pH 

as low as 6 standard units (fig. 6 and appendix 1). 
Assessment of Water Quality by Comparisons to Regulatory Standards 23

Table 4. Number of samples that exceeded designated-use water-quality standards at surface-water quality monitoring stations in the 

Central Arizona Basins study area, through 1998

[Designated uses and the numeric water-quality standards are from State of Arizona (1996), except for the domestic water source Secondary Maximum 

Contaminant Level for dissolved solids, which is from the U.S. Environmental Protection Agency (1996). Stations were evaluated on the basis of samples 

collected over the period of record, and in some cases exceedences occur for samples collected before the standards were established. Exceedences are expressed 

as a fraction of the total number of samples analyzed for that constituent. SMCL, Secondary Maximum Contaminant Level; mg/L, milligrams per liter; >, greater 

than; <, less than; ---, the stream reach for the station is not designated for this use, or data are not available for evaluation] 

Designated use for 

water body:

pH Dissolved oxygen

Dissolved 

solids Ammonia Nitrate

Aquatic 

and 

wildlife, 

full and 

partial 

body 

contact

Domestic 

water 

source

Agricultural Aquatic and wildlife

Domestic 

water 

source 

SMCL

Aquatic and wildlife

Domestic 

water 

Irrigation source 

Livestock 

watering

Warmwater 

fishery

Coldwater 

fishery

Effluent 

dependent

Warmwater 

fishery

Coldwater 

fishery

Range of values in 

compliance with 

standard:

>6.5 and 

<9.0 

standard 

pH units

>5.0 and 

<9.0 

standard 

pH units

>4.5 and 

<9.0 

standard 

pH units

>6.5 and 

<9.0 

standard 

pH units >6.0 mg/L >7.0 mg/L

>1.0 mg/L1

>3.0 mg/L2

<500 

mg/L

Varies by stream 

temperature and pH <10 mg/L

Central Highlands

Black River 2/41 2/41 2/41 2/41 --- 2/35 --- 0/44 --- --- 0/44

White River 3,4 4/44 3/44 3/44 4/44 --- 2/37 --- 0/44 --- --- 0/44

Pinal Creek 3/158 --- --- 3/158 3/154 --- --- --- 0/91 --- ---

Salt River near 

Roosevelt

1/264 --- 1/264 1/264 2/240 --- --- --- 0/156 --- ---

Tonto Creek 0/55 --- 0/55 0/55 --- 4/40 --- --- --- --- ---

Salt River below 

Stewart Mountain 

Dam

3/496 2/496 2/496 3/496 --- 27/119 --- 249/389 --- 0/117 0/163

Verde River near 

Clarkdale

0/188 --- 0/188 0/188 1/176 --- --- --- 0/105 --- ---

Verde River near 

Cornville

0/24 --- 0/24 0/24 0/20 --- --- --- --- --- ---

Oak Creek near 

Sedona

0/20 0/20 0/20 0/20 --- 0/19 --- 0/23 --- --- 0/21

Oak Creek at Red 

Rock Crossing

0/141 0/141 0/141 0/141 --- 0/138 --- 0/116 --- 0/77 0/127

Oak Creek at Cornville 0/35 0/35 0/35 0/35 --- 2/24 --- 0/34 --- --- 0/32

Verde River above 

West Clear Creek

0/21 --- 0/21 0/21 0/20 --- --- --- 0/20 0/20 ---

West Clear Creek 0/47 --- --- 0/47 --- 0/38 --- --- --- 0/39 ---

Verde River near Camp 

Verde

2/108 --- 1/108 2/108 2/98 --- --- --- 0/26 --- ---

East Verde River 0/46 0/46 0/46 0/46 1/46 --- --- 0/47 0/42 --- 0/42

Wet Bottom Creek 2/169 --- 1/169 2/169 4/117 --- --- --- 0/80 --- ---

Verde River below 

Tangle Creek

2/198 --- 1/198 2/198 0/191 --- --- --- 0/159 --- ---

Verde River below 

Bartlett Dam

0/603 0/603 0/603 0/603 0/115 --- --- 1/429 0/115 --- 0/185

Turkey Creek 0/37 --- 0/37 0/37 0/5 --- --- --- 0/25 --- ---

Agua Fria River near 

Rock Springs

0/166 0/166 0/166 0/166 9/162 --- --- 6/162 0/63 --- 0/166

Agua Fria River below 

Waddell Dam

0/75 --- --- 0/75 --- --- --- --- --- --- ---

See footnotes at end of table.
24 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Table 4. Number of samples that exceeded designated-use water-quality standards at surface-water quality monitoring stations in

the Central Arizona Basins study area, through 1998—Continued 

Designated use for 

stream reach where 

station is located:

pH Dissolved oxygen

Dissolved 

solids Ammonia Nitrate

Aquatic 

and 

wildlife, 

full and 

partial 

body 

contact

Domestic 

water 

source

Agricultural Aquatic and wildlife

Domestic 

water 

source 

SMCL

Aquatic and wildlife

Domestic 

water 

Irrigation source

Livestock 

watering

Warmwater 

fishery

Coldwater 

fishery

Effluent 

dependent

Warmwater 

fishery

Coldwater 

fishery

Range of values in 

compliance with 

standard:

>6.5 and 

<9.0 

standard

pH units

>5.0 and 

<9.0 

standard 

pH units

>4.5 and 

<9.0 

standard 

pH units

>6.5 and 

<9.0 

standard 

pH units >6.0 mg/L >7.0 mg/L

>1.0 mg/L1

>3.0 mg/L2

<500 

mg/L

Varies by stream 

temperature and pH <10 mg/L

Basin and Range Lowlands

Central Arizona 

Project Canal near 

Parker

--- 0/41 0/41 0/41 --- --- --- 39/41 --- --- 0/140

Central Arizona 

Project Canal at 

Phoenix

--- 0/123 0/123 0/123 --- --- --- 111/123 --- --- 0/123

Gila River at 

Winkelman

0/85 --- 0/85 0/85 2/77 --- --- --- 0/8 --- ---

San Pedro River at 

Charleston

0/87 --- 0/87 0/87 5/85 --- --- --- 0/83 --- ---

San Pedro River below 

Aravaipa Creek

0/38 --- --- 0/38 3/37 --- --- --- 0/35 --- ---

San Pedro River at 

Winkelman

0/38 --- --- 0/38 1/22 --- --- --- 0/16 --- ---

Gila River at Kelvin 10/996 --- 6/996 10/996 2/68 --- --- --- 0/51 --- ---

Santa Cruz River at 

Rio Rico

0/26 --- --- 0/26 --- --- 0/26 --- --- --- ---

Santa Cruz River at 

Tubac

0/28 --- --- 0/28 --- --- 0/27 --- --- --- ---

Santa Cruz River at 

Cortaro

0/13 --- --- --- --- --- 6/12 --- --- --- ---

91st Avenue 

WastewaterTreatment Plant 

outfall4

0/24 --- 0/24 --- --- --- 0/24 --- --- --- ---

Gila River at Buckeye 

Canal4

--- --- 0/40 0/40 --- --- --- --- --- --- ---

Hassayampa River4 1/49 --- 1/49 1/49 --- --- 0/43 --- --- --- ---

Gila River at Gillespie 

Dam

5/646 --- 4/646 5/646 --- --- 0/222 --- --- --- ---

1Applies from sunset to 3 hours after sunrise. If greater than 90 percent saturation, then a lower concentration is acceptable. 2Applies from 3 hours after sunrise to sunset. If greater than 90 percent saturation, then a lower concentration is acceptable. 3The standard does not apply legally because the station is on Native American land, but the standard was used in this study to identify river reaches with impaired water quality. 4Designated use determined on the basis of other nearby stream reaches or known water uses.
Assessment of Water Quality by Comparisons to Regulatory Standards 25

Table 5. Number of samples that exceeded stream-reach specific water-quality standards for total nitrogen and total phosphorus

concentrations at surface-water quality monitoring stations in the Central Arizona Basins study area, through 1998

[Standards are from State of Arizona (1996). Stations were evaluated on the basis of samples collected over the period of record, and in some cases

exceedences occur for samples collected before the standards were established. mg/L, milligrams per liter; N, total nitrogen; P, total phosphorus;

---, no standard]

Station

 Number of years in which 

annual mean concentration 

exceeded criterion/number 

of years evaluated

 90th percentile for 

concentration,1

in mg/L

 Number of samples that 

exceeded single-sample 

maximum criterion/number 

of samples analyzed

N P N P N P

Black River, Tonto Creek, and their tributaries

Concentration not to be exceeded, in mg/L 0.50 0.10 1.00 0.20 2.00 0.80

Black River

White River2

Tonto Creek

0/4

1/4

2/6

0/4

1/4

0/6

0.56

0.95

0.78

0.12

0.15

0.06

0/43

0/43

1/54

0/43

0/43

0/54

Salt River and tributaries above Theodore Roosevelt Lake, excluding the Black River, the White River, and Pinal Creek 

Concentration not to be exceeded, in mg/L 0.60 0.12 1.20 0.30 2.00 1.0

Salt River near Roosevelt 12/23 10/23 1.15 0.25 9/256 3/260

Salt River, from Stewart Mountain Dam to the confluence with the Verde River

Concentration not to be exceeded, in mg/L 0.60 0.05 --- --- 3.0 0.20

Salt River below Stewart Mountain Dam 6/19 4/19 0.84 0.07 1/144 1/145

Oak Creek, including the West Fork of Oak Creek

Concentration not to be exceeded, in mg/L 1.0 0.10 1.50 0.25 2.50 0.30

Oak Creek near Sedona

Oak Creek at Red Rock Crossing

Oak Creek near Cornville

0/3

0/13

0/3

0/3

0/13

0/3

0.74

0.80

0.70

0.10

0.07

0.10

0/22

2/131

0/29

0/22

1/141

1/30

Verde River and its tributaries, excluding Oak Creek, from the headwaters to Bartlett Lake

Concentration not to be exceeded, in mg/L 1.0 0.10 1.50 0.30 3.00 1.00

Verde River near Clarkdale

Verde River near Cornville

Verde River above West Clear Creek

West Clear Creek

Verde River near Camp Verde

East Verde River

Wet Bottom Creek

Verde River above Tangle Creek

1/19

0/3

0/3

0/3

1/13

0/9

0/18

0/18

3/19

3/3

0/3

0/3

4/13

0/9

0/18

4/18

0.90

0.76

0.63

0.39

1.06

0.42

0.70

0.90

0.17

30.35

0.15

0.09

0.19

0.04

0.04

0.13

2/178

0/23

0/20

0/39

0/95

0/42

0/76

2/191

2/188

0/23

0/20

0/39

1/104

0/47

0/100

0/197

1Number listed is the 90th percentile concentration for all samples collected at that station.

2The standard does not apply legally because the station is on Native American land, but the standard was used in this study to identify river reaches with impaired water quality. 3Exceeds criterion.
26 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

46

300

0

100

200

DISSOLVED OXYGEN,

AS PERCENT SATURATION

20

0

10

DISSOLVED OXYGEN,

IN MILLIGRAMS PER LITER

41 44 188 46 158 55 141 264 21 47 169 198 37 496 24 35 20 108 166 603 75

35 37 176 46 154 40 138 240 20 38 117 191 5 119 20 24 19 98 163 115 73

5

10

6

7

8

9

pH,

IN STANDARD UNITS

44 45 192 47 160 55 147 265 21 46 181 199 40 173 24 34 23 112 167 191 75

40

0

20

10

30

STREAM TEMPERATURE,

IN DEGREES CELSIUS

120 12 98 181 20 38 52 170 2 71 123 58 155 69 66

+

+++

+

+

+

ALKALINE

SUPERSATURATED SALT RIVER BELOW STEWART MTN DAM

BLACK RIVER

WHITE RIVER

PINAL CREEK

SALT RIVER NEAR

ROOSEVELT

TONTO CREEK

VERDE RIVER

NEAR CLARKDALE

VERDE RIVER

NEAR CORNVILLE

OAK CREEK

NEAR SEDONA

OAK CREEK AT RED

ROCK CROSSING

OAK CREEK

NEAR CORNVILLE

VERDE R. ABOVE

WEST CLEAR CR.

VERDE RIVER NEAR

WEST CLEAR

CAMP VERDE

CREEK

EAST VERDE RIVER

VERDE R. BELOW

TANGLE CREEK

VERDE R. BELOW

BARTLETT DAM

TURKEY CREEK

AGUA FRIA R. NEAR

ROCK SPRINGS

WET BOTTOM

CREEK

AGUA FRIA R. BLW

WADDELL DAM

ACIDIC

UNDERSATURATED

Figure 6. Distributions of stream properties and water-chemistry constituent concentrations at surface-water quality 

monitoring stations in the Central Arizona Basins study area, through 1998. CENTRAL HIGHLANDS. Explanation on page 32.
Assessment of Water Quality by Comparisons to Regulatory Standards 27

CAP* CANAL NEAR

PARKER

CAP* CANALAT PHOENIX

GILA RIVER

AT WINKELMAN

SAN PEDRO RIVER

AT CHARLESTON

SAN PEDRO RIVER

BLW ARAVAIPA CR.

SAN PEDRO RIVER

AT WINKELMAN

GILA RIVER

AT KELVIN

SANTA CRUZ RIVER

AT RIO RICO

SANTA CRUZ RIVER

AT TUBAC

SANTA CRUZ RIVER

AT CORTARO

SANTA CRUZ RIVER

AT LAVEEN

INDIAN BEND WASH

AT CURRY ROAD

BOX CULVERT AT

48th STREET DRAIN

27th AVENUE

AT SALT RIVER

43rd AVENUE AND

PEORIA AVENUE

OLIVE AVENUE

AND 67th AVENUE

91st AVENUE

WASTEWATER-TP

HASSAYAMPA

RIVER

GILA RIVER AT

GILLESPIE DAM

GILA RIVER AT

BUCKEYE CANAL

41 123 19 25 28 40 86 16 48 176 114 115 305 26 27 14 25 18 24 280

41 122 12 19 19 40 77 14 43 85 22 37 68 20 27 12 17 13 24 222

41 121 23 83 17 17 158 27 27 12 12 19 19 39 13 23 42

41 123 19 26 28 40 85 15 49 87 38 38 996 26 28 13 25 19 24 646

NUMBER OF SAMPLES

0

100

200

300

20

0

10

5

10

6

7

8

9

40

0

20

10

30

DISSOLVED OXYGEN,

AS PERCENT SATURATION

DISSOLVED OXYGEN,

IN MILLIGRAMS PER LITER

pH,

IN STANDARD UNITS

STREAM TEMPERATURE,

IN DEGREES CELSIUS

ALKALINE

SUPERSATURATED

ACIDIC

UNDERSATURATED

Figure 6. Continued. BASIN AND RANGE LOWLANDS. *Central Arizona Project. Explanation on page 32.
28 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

WRIR034063 FIG. 1(P.4) WEB

43 43 47

DISSOLVED ORTHOPHOSPHATE,

0.01

IN MILLIGRAMS PER LITER

AS PHOSPHORUS

TOTAL PHOSPHORUS,

IN MILLIGRAMS PER LITER

AS PHOSPHORUS

9 11 136 44 133 24 127 208 21 36 118 176 42 85 20 70 163 136 75

37 37 51 38 165 63 49 29 143 14 44 107 86 72 23 31 18 87 75

0.1

10

10

1

100

44 44 167 47 152 54 116 220 21 46 172 159 25 389 23 34 23 81 162 429 75

10,000

10,000

100,000

1,000

1,000

100

DISSOLVED SOLIDS,

IN MILLIGRAMS PER LITER

SUSPENDED SEDIMENT,

IN MILLIGRAMS PER LITER

NUMBER OF SAMPLES

143 54 260 20 145 188 23 22 39 141 30 197 147 104 100 43 166 75

0.1

10

1

100

0.01

0.1

10

1

100

SALT RIVER BELOW

STEWART MTN DAM

BLACK RIVER

WHITE RIVER

PINAL CREEK

SALT RIVER NEAR

ROOSEVELT

TONTO CREEK

VERDE RIVER

NEAR CLARKDALE

VERDE RIVER

NEAR CORNVILLE

OAK CREEK

NEAR SEDONA

OAK CREEK AT RED

ROCK CROSSING

OAK CREEK

NEAR CORNVILLE

VERDE R. ABOVE

WEST CLEAR CR.

VERDE RIVER NEAR

WEST CLEAR

CAMP VERDE

CREEK

EAST VERDE RIVER

VERDE R. BELOW

TANGLE CREEK

VERDE R. BELOW

BARTLETT DAM

TURKEY CREEK

AGUA FRIA R. NEAR

ROCK SPRINGS

WET BOTTOM

CREEK

AGUA FRIA R. BLW

WADDELL DAM

Figure 6. Continued. CENTRAL HIGHLANDS. Explanation on page 32.
Assessment of Water Quality by Comparisons to Regulatory Standards 29

+

0.01

0.1

10

10

1

100

10,000

10,000

1,000,000

100,000

1,000

1,000

100

0.1

2

5

10

1

0.5

0.05

0.2

0.02

0.01

0.1

10

1

100

NUMBER OF SAMPLES

CAP* CANAL NEAR

PARKER

CAP* CANALAT PHOENIX

GILA RIVER

AT WINKELMAN

SAN PEDRO RIVER

AT CHARLESTON

SAN PEDRO RIVER

BLW ARAVAIPA CR.

SAN PEDRO RIVER

AT WINKELMAN

GILA RIVER

AT KELVIN

SANTA CRUZ RIVER

AT RIO RICO

SANTA CRUZ RIVER

AT TUBAC

SANTA CRUZ RIVER

AT CORTARO

SANTA CRUZ RIVER

AT LAVEEN

INDIAN BEND WASH

AT CURRY ROAD

BOX CULVERT AT

48th STREET DRAIN

27th AVENUE

AT SALT RIVER

43rd AVENUE AND

PEORIA AVENUE

OLIVE AVENUE

AND 67th AVENUE

91st AVENUE

WASTEWATER-TP

HASSAYAMPA

RIVER

GILA RIVER AT

GILLESPIE DAM

GILA RIVER AT

BUCKEYE CANAL

41 123 50 83 37 40 406 26 13 27 15 19 23 26 24 19 24 40 48 384

41 123 86 86 37 40 92 26 12 26 15 19 23 26 25 19 23 39 48 250

39 123 40 74 31 30 182 21 12 27 1 24 40 48 168

116 56 152 132 102 313 5 14 23 14 20 31 38 250

DISSOLVED ORTHOPHOSPHATE,

IN MILLIGRAMS PER LITER

AS PHOSPHORUS

TOTAL PHOSPHORUS,

IN MILLIGRAMS PER LITER

AS PHOSPHORUS

DISSOLVED SOLIDS,

IN MILLIGRAMS PER LITER

SUSPENDED SEDIMENT,

IN MILLIGRAMS PER LITER

+

++

++

Figure 6. Continued. BASIN AND RANGE LOWLANDS. *Central Arizona Project. Explanation on page 32.
30 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

0.01

0.02

0.05

TOTAL AMMONIA

PLUS ORGANIC NITROGEN,

IN MILLIGRAMS PER LITER

AS NITROGEN

NITRATE,

IN MILLIGRAMS PER LITER

AS NITROGEN

TOTAL NITROGEN,

IN MILLIGRAMS PER LITER

AS NITROGEN

0.01

0.05

0.01

0.02

0.1

10

0.5

1

1

0.2

0.1

AMMONIA,

IN MILLIGRAMS PER LITER

AS NITROGEN

NUMBER OF SAMPLES

0.1

0.2

0.5

10

1

5

2

50

20

0.02

0.05

0.1

0.2

0.5

10

1

5

2

50

20

0.01

44 44 150 254 54 163 179 23 21 127 32 20 44 95 42 142 188 185 25 166 75

91 156 117 105 77 20 39 26 42 76 147 115 25 63 35

43 43 142 260 53 145 187 23 22 141 29 20 39 105 47 76 197 144 25 166 75

43 43 138 256 54 144 178 23 22 131 29 20 39 95 42 76 191 144 25 166 75 SALT RIVER BELOW STEWART MTN DAM BLACK RIVER WHITE RIVER PINAL CREEK SALT RIVER NEAR ROOSEVELT TONTO CREEK VERDE RIVER NEAR CLARKDALE VERDE RIVER NEAR CORNVILLE OAK CREEK NEAR SEDONA OAK CREEK AT RED ROCK CROSSING OAK CREEK NEAR CORNVILLE VERDE R. ABOVE WEST CLEAR CR. VERDE RIVER NEAR WEST CLEAR CAMP VERDE CREEK EAST VERDE RIVER VERDE R. BELOW TANGLE CREEK VERDE R. BELOW BARTLETT DAM TURKEY CREEK AGUA FRIA R. NEAR ROCK SPRINGS WET BOTTOM CREEK AGUA FRIA R. BLW WADDELL DAM

+

Figure 6. Continued. CENTRAL HIGHLANDS. Explanation on page 32.
Assessment of Water Quality by Comparisons to Regulatory Standards 31

TOTAL AMMONIA

PLUS ORGANIC NITROGEN,

IN MILLIGRAMS PER LITER

AS NITROGEN

NITRATE,

IN MILLIGRAMS PER LITER

AS NITROGEN

TOTAL NITROGEN,

IN MILLIGRAMS PER LITER

AS NITROGEN

AMMONIA,

IN MILLIGRAMS PER LITER

AS NITROGEN

0.1

0.2

0.5

20

2

1

10

5

100

50

0.2

0.5

1

2

5

100

10

50

20

500

200

0.1

0.01

0.1

10

1

100 39 81 8 83 35 16 51 4 12 27 14 19 23 26 25 19 24 40 48 203

0.01

0.1

10

1

100 40 123 82 82 37 37 200 25 12 27 15 19 23 26 25 19 24 40 48 293 CAP* CANAL NEAR PARKER CAP* CANAL AT PHOENIX GILA RIVER AT WINKELMAN SAN PEDRO RIVER AT CHARLESTON SAN PEDRO RIVER BLW ARAVAIPA CR. SAN PEDRO RIVER AT WINKELMAN GILA RIVER AT KELVIN SANTA CRUZ RIVER AT RIO RICO SANTA CRUZ RIVER AT TUBAC SANTA CRUZ RIVER AT CORTARO SANTA CRUZ RIVER AT LAVEEN INDIAN BEND WASH AT CURRY ROAD BOX CULVERT AT 48th STREET DRAIN 27th AVENUE AT SALT RIVER 43rd AVENUE AND PEORIA AVENUE OLIVE AVENUE AND 67th AVENUE 91st AVENUE WASTEWATER-TP HASSAYAMPA RIVER GILA RIVER AT GILLESPIE DAM GILA RIVER AT BUCKEYE CANAL

39 123 81 81 37 34 87 23 12 26 12 19 23 26 25 18 24 40 48 284

40 123 81 86 37 34 85 23 12 26 12 24 19 40 48 244 23 26 25 18

NUMBER OF SAMPLES

+

+

+++

+

+

Figure 6. Continued. BASIN AND RANGE LOWLANDS. *Central Arizona Project. Explanation on page 32.
32 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

The State of Arizona promulgates four sets of

designated-use standards that require pH to be within a 

specified range (table 4; State of Arizona, 1996): 

domestic water sources (between 5.0 and 9.0 standard 

pH units); aquatic and wildlife, full and partial body 

contact (between 6.5 and 9.0); agricultural irrigation 

(between 4.5 and 9.0); and agricultural livestock 

watering (between 6.5 and 9.0). Given that the 

standards require that pH be within a specified range, 

the term “exceedence” is used in this report when pH is 

either above or below the range. Overall, surface-water 

resources in the study area generally met the waterquality standards for pH. The pH standard was not 

exceeded in any samples for 8 out of the 11 stations on 

reaches designated as a domestic drinking water 

source, and pH was higher than allowed by the standard 

in only a few samples at the White River, the Black 

River, and the Salt River below Stewart Mountain Dam. 

At 21 of 32 stations having established pH standards, 

there were no samples that exceeded the standards for 

aquatic and wildlife, and full and partial body contact 

designated uses. Of the stations that had one or more 

samples exceeding the standard, only the Black River 

and the White River had more than 5 percent of the 

total samples exceed the standards. Of the 27 stations 

on streams designated for agricultural irrigation, 10 had 

one or more samples exceed the irrigation waterquality standards, but of these stations, most had only 

1 or 2 percent of the total samples exceed standards. Of 

the 33 stations on streams designated for agricultural 

livestock watering, 11 stations had one or more 

samples exceed the standards. However, of these

11 stations, only the Black River and the White River 

had more than 5 percent of the total samples exceed the 

standards.

Dissolved oxygen is consumed or produced in 

many chemical reactions and is necessary for

respiration and photosynthesis of aquatic biota. 

Minimum levels of dissolved oxygen in the water are 

required for the survival of fish and aquatic invertebrates. The concentration of dissolved oxygen in water 

is dependent on temperature and, owing to the equilibrium between oxygen in the air and the oxygen 

dissolved in the water, atmospheric pressure as well. 

For this reason, it is useful to report dissolved oxygen 

as a concentration and as percent saturation. The 

equilibrium (100 percent saturation) concentration of 

dissolved oxygen in water increases as water temperature decreases and as atmospheric pressure increases. 

Stream temperature and atmospheric pressure vary 

greatly throughout the study area because of the large 

variation in land-surface altitudes, and as a result, the 

equilibrium concentration for dissolved oxygen is 

spatially variable. By standardizing dissolved-oxygen 

concentration data as a percentage of the saturated 

concentration, samples collected at different stream 

temperatures and atmospheric pressures can be 

compared directly. Most samples with dissolvedoxygen concentration data also had the oxygen content 

expressed as a percentage of the saturation concentration; however, at eight stations the percent of saturation 

was not computed for any samples. Concentrations of 

dissolved oxygen in the study area typically were 

between 8 and 11 mg/L and between 80 and 120 

percent saturation (fig. 6 and appendix 1). Exceptions 

to this are several samples from stations on effluentdependent and unregulated urban ephemeral reaches 

that typically were below 100 percent saturation and 

as low as 27 percent saturation for the Santa Cruz River 

at Cortaro Road (fig. 6 and appendix 1). 

The State of Arizona dissolved-oxygen standards 

for the three subcategories of aquatic and wildlife uses 

are (State of Arizona, 1996): warm water fisheries 

(minimum of 6.0 mg/L), cold water fisheries (minimum of 7.0 mg/L), and effluent-dependent stream 

reaches (minimum of 1.0 mg/L from sunset to 3 hours 

after sunrise, or a minimum of 3.0 mg/L otherwise). 

90th PERCENTILE

75th PERCENTILE

MORE THAN THE 90th PERCENTILE

25th PERCENTILE

10th PERCENTILE

MEDIAN (50th PERCENTILE)

LESS THAN THE 10th PERCENTILE

EXPLANATION FOR FIGURE 6

+ DATA POINT—n < 10

Effluent-dependent

Unregulated urban ephemeral

SURFACE-WATER QUALITY MONITORING

STATION—Color-coded by type of reach

Regulated

Unregulated intermittent

Unregulated perennial
Assessment of Water Quality by Comparisons to Regulatory Standards 33

Concentrations of dissolved oxygen lower than these 

standard values on a regular basis indicate that a stream 

may not be able to support aquatic life. Concentrations 

of dissolved oxygen in the study area generally met 

the standards for aquatic and wildlife uses (table 4). 

Twelve of the 17 stations on warm water fishery 

reaches had dissolved-oxygen concentrations lower 

than the standard; however, of these 12 stations, only 

the Agua Fria River near Rock Springs and the 

3 stations on the San Pedro River had concentrations 

lower than the standard in more than 5 percent of the 

total samples. Five of the 8 stations on cold water 

fishery reaches had one or more samples with 

concentrations of dissolved oxygen lower than the 

standard. Of these 5 stations, the Salt River below 

Stewart Mountain Dam had the highest percentage of 

total samples with dissolved-oxygen concentrations 

lower than the standard—about 20 percent. Of the 

6 stations on effluent-dependent reaches, only the 

Santa Cruz River at Cortaro had samples with

dissolved-oxygen concentrations lower than the 

standards; however, half of the total samples collected 

there had concentrations lower than the standard. 

Dissolved solids and suspended sediment

Dissolved solids are inorganic and organic 

materials dissolved in the water, and by mass they are 

composed mostly of common anions and cations. 

Common anions include bicarbonate, chloride, sulfate, 

and nitrate; common cations include calcium, magnesium, sodium, and potassium. Weathered rock and 

atmospheric deposition are natural sources of these 

ions; agricultural, industrial, and urban activities are 

anthropogenic sources. Water having high concentrations of dissolved solids can have an unpleasant taste, 

can cause salt to deposit in pipes and household 

appliances, and can result in salinized soil in irrigated 

areas. For a water supply to be well suited for a particular use, concentrations of dissolved solids should be 

within certain tolerances (table 6; Hem, 1985, p. 212–

221). Concentrations of dissolved solids are highly 

variable in the surface-water resources of the study area 

and ranged from 18 mg/L at 43rd Avenue and Peoria 

Avenue to nearly 8,000 mg/L at the Gila River at 

Gillespie Dam (fig. 6 and appendix 1). 

The USEPA Secondary Maximum Contaminant 

Level5 (SMCL) for dissolved solids in drinking water

is 500 mg/L (U.S. Environmental Protection Agency, 

1996). Five out of the 11 stations that are on domestic 

water source stream reaches had one or more samples 

with concentrations higher than the SMCL (table 4). 

Although less than 5 percent of the samples from 

the Agua Fria River near Rock Springs and the 

5An SMCL for a constituent is considered recommended, but 

nonenforceable, by Federal Guidelines under the Safe Drinking Water Act 

of 1986.

Table 6. Recommended upper limits of dissolved-solids 

concentrations for selected water uses

[Data from table 25, an unnumbered table on page 213, and table 29 in 

Hem, 1985. mg/L, milligrams per liter]

Use

Upper limit for 

dissolved solids 

concentration, in 

mg/L

Original 

source of data

Drinking water

Humans 500 (1)

Livestock:

Poultry 2,860 (2)

Pigs 4,290 (2)

Horses 6,435 (2)

Cattle (dairy) 7,150 (2)

Cattle (beef) 10,100 (2)

Sheep (adult) 12,900 (2)

Industrial uses

Boiler feedwater pressure 

(pounds per square inch)

0–150 700 (3)

150–700 500 (3)

700–1,500 200 (3)

1,500–5,000 0.5 (3)

Wood chemicals 1,000 (3)

Petroleum products 1,000 (3)

Canned, dried, and frozen 

fruits and vegetables

5 (3)

Soft drinks bottling 10 (3)

Leather tanning (finishing 

process)

5 (3)

Original data sources:

1U.S. Public Health Service (1962).

2McKee and Wolf (1963).

3U.S. Federal Water Pollution Control Administration (1968).
34 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Verde River below Bartlett Dam had concentrations 

higher than the SMCL, more than half of the samples 

collected on the Salt River below Stewart Mountain 

Dam and nearly all of the samples collected from the 

two stations on the Central Arizona Project Canal had 

concentrations higher than the SMCL. As a result of 

the high concentrations of dissolved solids in the 

Central Arizona Project Canal and the Salt River, water 

from these two sources typically would have to be 

blended with water from other sources that have low 

dissolved-solids concentrations, such as the Verde 

River or ground water, in order to meet the SMCL for 

drinking water. 

Suspended sediment is also a product of weathering and consists of solid inorganic and organic particles 

carried by streams. Phosphorus, certain metals, and 

hydrophobic organic compounds adsorb to sediment 

and, therefore, can be transported downstream along 

with the sediment in the suspended load and the 

bedload. Suspended-sediment concentration data were 

available for 33 monitoring stations and are highly 

variable (fig. 6). Concentrations typically range over 

several orders of magnitude at any given station and 

range from 1 mg/L or less at many stations to more 

than 100,000 mg/L at the San Pedro River at 

Charleston, the San Pedro River at Winkelman, and 

the Gila River at Kelvin. There are no water-quality 

standards for suspended-sediment concentration.

Nutrients

Nitrogen and phosphorus are important nutrients 

for terrestrial and aquatic plant and animal growth, and 

occur as several chemical species. Nitrogen in water 

generally occurs as ammonium (NH4

+), un-ionized 

ammonia (NH3), nitrate (NO3

-

), nitrite (NO2

-

), and as 

a component of organic molecules. Phosphorus in 

water generally occurs in the dissolved phase as 

ortho-phosphate, as a component in an organic 

compound, or as phosphate sorbed to sediment (Hem, 

1985). Dissolved orthophosphate consists of the ion 

PO4

3- in various combinations with hydrogen: H3PO4, 

H2PO4

-

, HPO4

2-, and PO43- (Hem, 1985). Nitrogen and 

phosphorus can be introduced to streams through

several processes. Nitrogen oxides in the atmosphere, 

from lightning or from combusted fossil fuels, can be 

scavenged by rainfall and transported directly in runoff. 

Nitrogen gas (N2) can be converted to other forms of 

nitrogen by certain algae and bacteria, and these forms 

can be transported to streams. Phosphorus is a common 

element in igneous rocks and sediment, and can occur 

in abundance in marine phosphorite deposits. Fertilizer, 

livestock waste, and human waste are rich sources of 

nitrogen and phosphorus that can be introduced to the 

hydrologic system through runoff processes or directly 

from wastewater-treatment facilities. Nutrients from 

these sources can be beneficial in water used for crop 

irrigation. 

Large influxes of nutrients to streams, however, 

cause eutrophication, the enrichment of water 

with nutrients, and lead to excessive algae growth. 

The excessive algae may clog pipes and water courses, 

require additional water treatment, or cause fish kills 

that result from depletion of oxygen when the algae 

decays and from the release of toxins from algae into 

the water. Excessive nitrate can lead to methemoglobinemia, or “blue baby syndrome,” a medical 

condition generally in infants less than 4 months old

in which nitrates prevent the transport of oxygen 

through the bloodstream to vital organs. 

Data for dissolved orthophosphate concentrations 

were available for 34 of the 41 stations and were

typically between 0.01 and 0.10 mg/L, except at 

stations on effluent-dependent reaches where concentrations were more variable and typically were between 

0.10 and 8.0 mg/L (fig. 6 and appendix 1). Data for 

total phosphorus were available for all 41 stations, and 

concentrations typically were between 0.01 and 1.0 

mg/L at stations on unregulated perennial and regulated 

reaches. At stations on unregulated intermittent, unregulated urban ephemeral, and effluent-dependent 

reaches, concentrations of total phosphorus were 

higher—as much as 53 mg/L at the Hassayampa River. 

There are no designated-use water-quality 

standards for phosphorus; however, there are streamreach specific water-quality standards for the Salt and 

Verde Rivers and their tributaries (State of Arizona, 

1996). The standards require that the annual mean 

concentration, the 90th-percentile concentration 

from all samples, and single sample maximum 

concentrations be less than an assigned value for 

specific reaches (table 5). The standards for annual 

mean total phosphorus concentration in most years 

were met at stations on the tributaries of the Salt and 

Verde Rivers, whereas stations on the main stems of the 

Salt and Verde Rivers tended to exceed these standards 

in several years (table 5). All stations met the standard 

for the 90th-percentile concentration for all samples 

(0.30 mg/L) except the Verde River near Cornville,
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 35

which had a 90th-percentile concentration of 0.35 

mg/L. Most stations met the standards for the single 

sample maximum concentration of total phosphorus. 

Six stations had one or more samples exceed these 

standards; however, the exceedences represented less 

than 5 percent of the total samples collected at each 

station. 

Concentrations of total nitrogen ranged from less 

than 0.10 mg/L at several stations to 120 mg/L at 

27th Avenue at Salt River (fig. 6 and appendix 1). 

Concentrations of nitrate, ammonia, total ammonia 

plus organic nitrogen, and total nitrogen tended to be 

lower and less variable at stations in the Central Highlands than at stations in the Basin and Range Lowlands. 

Distributions of nitrate and ammonia concentrations 

can be used to estimate the predominant nitrogen 

species. Nitrate and ammonia concentrations typically 

are much lower than total nitrogen concentrations at 

unregulated perennial, unregulated intermittent, and 

regulated stations. This difference indicates that most 

of the total nitrogen occurs as organic nitrogen at these 

stations (fig. 6 and appendix 1). In contrast, nitrate and 

(or) ammonia make up a significant portion of the total 

nitrogen at stations on unregulated urban ephemeral 

and effluent-dependent reaches (fig. 6 and appendix 1). 

For example, most of the total nitrogen occurs as 

ammonia at the Santa Cruz River at Cortaro. At the 

91st Avenue Wastewater-Treatment Plant outfall and at 

Indian Bend Wash, nitrate, ammonia, and organic nitrogen make up a significant portion of the total nitrogen. 

The State of Arizona has two designated-use waterquality standards for nitrogen species and three streamreach specific standards for total nitrogen in the Salt 

and Verde Rivers (State of Arizona, 1996). 

The standard for nitrate in streams designated as a 

domestic water source is 10 mg/L as nitrogen, which 

was not exceeded at the 11 stations on reaches having 

this designated use (table 4). The standard for ammonia

applies to streams with aquatic and wildlife warm 

water and cold water fisheries designated uses and 

varies by stream temperature and pH. No samples from 

stations on stream reaches having these designated uses 

exceeded the ammonia standard (table 4). Stream-reach 

specific standards for total nitrogen in the Salt and 

Verde Rivers and their tributaries are similar to those 

for phosphorus and consist of the annual mean concentration, the 90th-percentile concentration from all samples, and single sample maximum concentration

(table 5). The standard for the annual mean total

nitrogen concentration was exceeded frequently at the 

Salt River near Roosevelt and the Salt River below 

Stewart Mountain Dam, but was exceeded infrequently 

at stations on the Verde River and its tributaries 

(table 5). Although total nitrogen concentrations from 

stations on the Salt River and its tributaries do not 

appear much different from those of the Verde River 

(fig. 6), the standard was exceeded more often in the 

Salt River and its tributaries than in the Verde River and 

its tributaries. This difference results from the standard 

being set at a higher concentration for reaches on the 

Verde River and its tributaries (1.0 mg/L) than for 

reaches on the Salt River and its tributaries (0.50–

0.60 mg/L). The standard for the 90th-percentile 

concentration of total nitrogen for all samples was not 

exceeded at any of the stations sampled. Most stations 

met the standards for the single sample maximum 

concentration of total nitrogen. Six stations had one or 

more samples exceed these standards; however, the 

exceedences represented less than 5 percent of the total 

samples collected at each station.

RELATION OF STREAM PROPERTIES AND WATERCHEMISTRY CONSTITUENT CONCENTRATIONS 

TO NATURAL AND HUMAN FACTORS

Stream properties and water-chemistry constituent 

concentrations were found to be related to streamflow, 

season, water management, stream permanence, land 

use, and water use. An understanding of how natural 

and human factors affect water-quality can be used to 

build a regional conceptual model of the water quality 

of streams and reservoirs in the CAZB study area. 

In addition, an understanding of how these natural and 

human factors affect water-quality is needed for design 

and evaluation of data from water-quality monitoring 

networks. For instance, if concentrations of a given 

constituent are related to streamflow, then the sample 

collection design should take into account variations in 

streamflow, and streamflow at the time of sampling 

should be considered when evaluating the water-quality 

data.

Streamflow and Season

Seasonal variation in the relative contribution of 

streamflow from different sources and seasonal 

variation in physical or biological processes in the 

stream cause seasonal variation in stream properties 

and water-chemistry constituent concentrations. For 
36 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

instance, precipitation (fig. 7) that becomes runoff and 

enters a stream can dilute or concentrate waterchemistry constituent concentration in the stream. 

Similar dilution or concentration processes can occur 

during periods of low flow as a result of changes in the 

relative contribution to flow to a stream from springs 

and ground-water sources that have different constituent concentrations. The relative contribution to the total 

streamflow from different sources typically is seasonal, 

and as a result, the concentrations of some constituents 

in streams vary seasonally. In addition, seasonal variation of concentrations also may result from seasonal 

variations in natural or human activities, such as microbiological consumption or production of constituents, 

or application of fertilizers during certain seasons.

The relation of the stream properties and waterchemistry constituent concentrations to streamflow at 

the 41 stations was determined by statistical correlations and also by graphical analysis for 6 stations. 

The strength and direction of the correlation between 

streamflow and the stream properties and waterchemistry constituent concentrations was determined 

for all stations using computer software (SPSS Inc., 

1997) to perform the Kendall’s tau test (Kendall, 1938). 

Kendall’s tau is a rank-based, nonparametric test that 

determines the monotonic relation between two 

variables and is suitable for data that are skewed, 

include censored data, and (or) include outliers (Helsel 

and Hirsch, 1992). If the relation is nonmonotonic, 

results indicate the predominant monotonic relation 

exhibited by the samples. The seasonal variations and 

relations to streamflow for stream properties and waterchemistry constituent concentrations were illustrated 

graphically by using data from six stations collected 

during water years 1996–98 to contrast how they vary 

for stations affected by different natural and human 

factors. In a few cases a relation is apparent in the 

graphs but was not detected by the Kendall’s tau test. 

This discrepancy occurred because the graphs are for 

1996–98, whereas the Kendall’s tau test was performed 

on all historic data inclusive of 1998. 

Unregulated Perennial and Intermittent Reaches

Correlations of the stream properties and waterchemistry constituent concentrations with streamflow 

were common and generally consistent in sign (positive 

or negative) at stations on unregulated perennial and 

intermittent reaches (table 7; San Pedro River at 

Charleston and Verde River below Tangle Creek in 

figure 8). Correlations between streamflow and stream 

temperature, streamflow and pH, streamflow and 

dissolved-oxygen saturation, and streamflow and 

dissolved-solids concentration were generally either 

negative or not significant. In contrast, correlations 

between streamflow and concentrations of dissolved 

oxygen, suspended sediment, and nutrients were 

generally either positive or not significant. 

Stream temperature was negatively correlated with 

streamflow because the samples from higher streamflows generally were collected during the winter or 

spring when stream temperatures were low as a result 

of lower air temperatures and (or) snowmelt runoff. 

In addition, the coinciding cold stream temperatures 

and high stream discharges also resulted in a positive 

correlation between dissolved-oxygen concentration 

and streamflow because the saturation equilibrium 

concentration for dissolved oxygen is higher for colder 

stream temperatures. The negative correlation between 

dissolved-oxygen percent saturation and streamflow at 

stations draining the Central Highlands resulted from 

streams being in equilibrium with atmospheric oxygen 

when storm runoff contributed flow and from streams 

being slightly oversaturated, probably because of 

increased algal production of oxygen, during periods of 

low flow. Dissolved-solids concentrations and pH were 

negatively correlated with streamflow because base 

flow in streams is diluted by runoff from precipitation 

that has a lower pH and lower dissolved-solids concentration (fig. 7; National Atmospheric Deposition 

Program, 1998a, 1998b). 

Concentrations of suspended sediment and 

nutrients were positively correlated with streamflow as 

a result of several runoff processes. Concentrations of 

nitrate were generally higher in precipitation (fig. 7; 

National Atmospheric Deposition Program, 1998a, 

1998b) than in streams during periods of low flow

(San Pedro River at Charleston and Verde River below 

Tangle Creek in figure 8). Therefore, as precipitation 

runoff entered the streams and increased the discharge, 

the nitrate concentration also increased. Concentrations 

of ammonia also were generally higher in precipitation 

than in streamflow during periods of low flow; 

however, there was generally no relation between 

concentration of ammonia and streamflow. As 

precipitation runoff flows over the land surface and 

through soils, soluble nutrients are dissolved into the 

runoff and particulate nutrients and sediment are 

entrained in the overland runoff and transported to the 

stream. These processes likely increased the concentrations of nutrients and suspended sediment in the 

stream. In addition, the increase in streamflow that 

results from runoff also entrains sediment and organic 

matter from the stream channel into suspension, 

thereby increasing the concentrations of suspended 

sediment and nutrients.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 37

Table 7. Correlation of stream properties and water-chemistry constituent concentrations with streamflow at surface-water quality

monitoring stations in the Central Arizona Basins study area, through 1998

[Correlations between streamflow and concentration were determined using the Kendall’s tau test and are reported as follows: +++, a strong positive correlation 

(p<0.01); ++, a positive correlation (p<0.05); +, a marginally positive correlation (p<0.10); 0, no significant correlation (p>0.10); -, a marginally negative

correlation (p<0.10); --, a negative correlation (p<0.05); ---, a strong negative correlation (p<0.01); n, not determined. The colors of the table cells indicate

the type of streamflow at the station; blue, unregulated perennial; green, unregulated intermittent; yellow, unregulated urban ephemeral; magenta, regulated;

orange, effluent dependent]

Station

TemperaturepHDissolved-oxygen concentrationDissolved oxygen,percent saturationDissolved-solids concentrationSuspended-sediment concentrationDissolved-orthophosphorus concentrationTotal phosphorus concentrationNitrate concentrationAmmonia concentrationTotal ammonia and organicnitrogen concentrationTotal nitrogen concentration

Central Highlands

Black River -- --- 0 n --- ++ + +++ +++ n 0 0

White River 0 --- 0 n --- +++ 0 ++ +++ n 0 0

Pinal Creek --- 0 +++ 0 --- +++ 0 +++ +++ + ++ +++

Salt River near Roosevelt --- --- +++ 0 --- +++ +++ +++ 0 0 ++ ++

Tonto Creek --- --- + -- --- 0 0 + +++ n 0 0

Salt River below Stewart 

Mountain Dam

0 0 0 ++ -- 0 + +++ 0 0 0 0

Verde River near Clarkdale --- 0 +++ --- --- ++ +++ +++ +++ 0 +++ +++

Verde River near Cornville --- 0 ++ n --- n 0 0 + n 0 0

Oak Creek near Sedona --- 0 +++ n --- 0 0 +++ 0 n 0 0

Oak Creek at Red Rock 

Crossing

--- --- +++ -- --- + +++ +++ + 0 0 ++

Oak Creek near Cornville --- - ++ n --- n 0 0 -- n 0 -

Verde River above West 

Clear Creek

--- 0 0 --- --- ++ 0 ++ 0 0 +++ 0

West Clear Creek --- --- +++ --- --- 0 +++ +++ 0 0 ++ ++

Verde River near Camp 

Verde

--- 0 +++ --- --- +++ 0 +++ +++ 0 0 0

East Verde River -- +++ 0 0 --- 0 n + 0 0 0 0

Wet Bottom Creek --- 0 +++ 0 --- +++ --- +++ 0 ++ 0 0

Verde River below Tangle 

Creek

--- 0 +++ --- --- +++ +++ +++ ++ + +++ +++

Verde River below Bartlett 

Dam

0 -- 0 0 --- 0 0 0 0 0 -- -

Turkey Creek --- 0 0 0 --- ++ n +++ 0 0 0 0

Agua Fria River near Rock 

Springs

--- + +++ 0 --- 0 +++ +++ +++ 0 0 +++

Agua Fria River below 

Waddell Dam

-- -- 0 0 0 0 +++ ++ ++ 0 0 0
38 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Table 7. Correlation of stream properties and water-chemistry constituent concentrations with streamflow at surface-water quality

monitoring stations in the Central Arizona Basins study area, through 1998—Continued

Station

TemperaturepHDissolved-oxygen concentrationDissolved oxygen,percent saturationDissolved-solids concentrationSuspended-sediment concentrationDissolved-orthophosphorus concentrationTotal phosphorus concentrationNitrate concentrationAmmonia concentrationTotal ammonia and organicnitrogen concentrationTotal nitrogen concentration

Basin and Range Lowlands

Central Arizona Project near 

Parker

0 0 0 0 ++ n 0 -- 0 0 - 0

Central Arizona Project at 

Phoenix

++ 0 --- 0 +++ 0 +++ ++ 0 0 --- ---

Gila River at Winkelman 0 0 -- --- --- +++ 0 +++ -- 0 + ++

San Pedro River at 

Charleston

--- 0 + - +++ +++ 0 0 0 0 0 +++

San Pedro River below 

Aravaipa

-- 0 0 0 --- +++ 0 +++ +++ 0 +++ +++

San Pedro River at 

Winkelman

0 0 0 n --- ++ 0 +++ ++ ++ ++ ++

Gila River at Kelvin 0 - 0 0 --- +++ ++ +++ ++ ++ +++ +++

Santa Cruz River at Rio 

Rico

0 0 0 n --- 0 --- --- + 0 --- ---

Santa Cruz River at Tubac -- 0 0 -- --- +++ 0 +++ + +++ +++ +++

Santa Cruz River at Cortaro 0 0 0 0 0 0 0 0 + 0 + +

Santa Cruz River at Laveen 0 0 0 n 0 +++ n 0 0 0 0 0

Indian Bend Wash at Curry 

Road

0 0 0 ++ 0nn 0 0 0 0 0

Box Culvert at 48th Street 

Drain

++ 0 0 + 0nn + 0 +++ 0 +

27th Avenue at Salt River 0 0 0 0 0nn 0 0 + 0 0

43rd Avenue and Peoria 

Avenue

0 0 0 0 0nn 0 0 0 - 0

Olive Avenue and 67th 

Avenue

0 0 0 0 0nn ++ 0 0 0 0

91st Avenue WastewaterTreatment Plant outfall

0 0 0 -- -- 0 0 0 0 0 0 ++

Gila River at Buckeye Canal ++ 0 0 0 0 0 0 0 0 0 + 0

Hassayampa River -- --- -- --- --- +++ 0 +++ --- 0 +++ ---

Gila River at Gillespie Dam -- --- 0 0 --- 0 ++ +++ --- +++ 0 ---
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 39

Unregulated perennial and intermittent streams 

typically received direct runoff during the summer 

from thunderstorms and during the winter from frontal 

storms. The monthly streamflow volumes, however, 

had a unimodal distribution because the volume of 

direct runoff usually was much larger for one season 

than the other (table 1). For streams in the Central 

Highlands, the mode of the streamflow volume 

distribution was centered between February and April, 

depending on the altitude of the basin, as a result of 

runoff from winter precipitation and spring snowmelt. 

In contrast, for streams in the southeastern part of the 

Basin and Range Lowlands, such as the San Pedro 

River, the mode of the monthly streamflow volume 

distribution was centered in August as a result of direct 

runoff predominately being generated by summer 

thunderstorms. 

Seasonal patterns of the stream properties and 

water-chemistry constituent concentrations in 

unregulated perennial and intermittent streams 

generally resulted from seasonal patterns in climate 

because the stream properties and water-chemistry 

constituent concentrations were correlated with streamflow (San Pedro River at Charleston and Verde River 

below Tangle Creek in figure 8). In general, stream 

temperature, pH, dissolved-oxygen percent saturation, 

and concentrations of dissolved solids were low during 

seasons that had substantial direct runoff (winter and 

summer) as compared to seasons that had long periods 

of low flow (spring and fall). In contrast, concentrations of suspended sediment and of nutrients generally 

were high during seasons that had substantial direct 

runoff (winter and summer). As previously discussed, 

seasonal patterns in dissolved-oxygen concentrations at 

some stations were related to the coincident seasonal 

patterns in stream temperature and streamflow.

Seasonal patterns of stream properties and waterchemistry constituent concentrations that are correlated 

with streamflow are much less pronounced during 

drought years (such as 1996) than during wet years 

(such as 1998). For instance, concentrations of dissolved solids, suspended sediment, total phosphorus 

and total nitrogen at the Verde River below Tangle 

Creek were much less variable during 1996 than during 

1998 (fig. 8)

4

6

5

EXPLANATION

10th PERCENTILE

25th PERCENTILE

MEDIAN

75th PERCENTILE

90th PERCENTILE

10th AND 90th PERCENTILES

DATA VALUES OUTSIDE THE

0

10

5

0

0.2

0.4

0.6

0

0.6

0.2

0.4

0

1

0.5

pH

DISSOLVED SOLIDS

AMMONIA AS NITROGEN

NITRATE AS NITROGEN

TOTAL NITROGEN

MILLIGRAMS PER LITER STANDARD UNITS

GRAND

CANYON

ORGAN

PIPE

OLIVER

KNOLL

Figure 7. Distributions of precipitation-volume weighted annual 

average pH, dissolved solids, ammonia, nitrate, and total nitrogen 

concentrations in precipitation at three National Atmospheric 

Deposition Program monitoring stations in Arizona, 1980–96. The 

number of observations for each station are: Grand Canyon, 16; 

Organ Pipe, 17; and Oliver Knoll, 16.
40 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

0

0

6

3024

1812

60

3024

1812

3024

1812

60

70

3528

2114

0

500400

300200

100

80

4032

2416

0

5040

3020

10

3528

2114

70

3024

1812

60

1995 1996

SAN PEDRO RIVER AT CHARLESTON (Unregulated perennial)

1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

0.01 0.10 1

IN DEGREES CELSIUS STREAM TEMPERATURE,

IN CUBIC METERS PER SECOND DISCHARGE,

IN DEGREES CELSIUS STREAM TEMPERATURE,

1995 1996

GILA RIVER AT KELVIN (Regulated)

1997 1998

0.10 10 1 100

1995 1996

VERDE RIVER BELOW TANGLE CREEK (Unregulated perennial)

1997 1998

1 100 10

Figure 8. Relation of stream properties and water-chemistry constituent concentrations to season and stream discharge, at selected surface-water quality 

monitoring stations in the Central Arizona Basins study area, October 1995 through September 1998. Solid white line is a locally weighted scatter plot smooth (Cleveland, 1979). m3/s, cubic meters per second.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 41

80

4032

2416

60

3024

1812

3024

1812

60

4032

2416

80

80

4032

2416

60

3024

1812

0

1.5

3

4.5

6

7.5

50

25

20

15

10

4032

2416

80

1995 1996

SANTA CRUZ RIVER AT TUBAC (Effluent-dependent)

1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

0.010 0.10 1 10

IN DEGREES CELSIUS STREAM TEMPERATURE,

IN CUBIC METERS PER SECOND DISCHARGE,

IN DEGREES CELSIUS STREAM TEMPERATURE, 1995 1996 91st AVENUE WASTEWATER-TREATMENT PLANT OUTFALL (Effluent-dependent) 1997 1998 1 10

1995 1996

HASSAYAMPA RIVER (Effluent-dependent)

1997 1998

0.10 1 10 100

Figure 8. Continued. m3/s, cubic meters per second.
42 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

80

4032

2416

0

5040

3020

10

98.5

87.5

76.5

0

98.5

87.5

76.5

6.5

7

7.5

8

8.5

9

6.5

7

7.5

8

8.5

9

6.5

7

7.5

8

8.5

9

0

500400

300200

100

98.5

87.5

76.5

1995 1996

SAN PEDRO RIVER AT CHARLESTON (Unregulated perennial)

1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

0.01 0.10 1

IN STANDARD UNITS pH,

IN CUBIC METERS PER SECOND DISCHARGE,

IN STANDARD UNITS pH,

1995 1996

GILA RIVER AT KELVIN (Regulated)

1997 1998

0.10 1 10 100

1995 1996

VERDE RIVER BELOW TANGLE CREEK (Unregulated perennial)

1997 1998

1 10 100

Figure 8. Continued. m3/s, cubic meters per second.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 43

7

7.56.5

8

8.5

9

7

7.56.5

8

8.5

9

7

7.56.5

8

8.5

9

5

0

2520

15

10

0

1.5

3

4.5

6

7.5

98.5

87.5

7

6.5

98.5

87.5

7

6.5

98.5

87.5

7

6.5

6

0

30

24

1812

1995 1996

SANTA CRUZ RIVER AT TUBAC (Effluent-dependent)

1997 1998

0.01 0.10 1 10

IN STANDARD UNITS pH,

IN CUBIC METERS PER SECOND DISCHARGE,

IN STANDARD UNITS pH,

1995 1996

91st AVENUE WASTEWATER-TREATMENT PLANT OUTFALL (Effluent-dependent)

1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

1 10

1995 1996

HASSAYAMPA RIVER (Effluent-dependent)

1997 1998

0.10 10 100 1

Figure 8. Continued. m3/s, cubic meters per second.
44 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

8

64

20

10

80

4032

2416

0

5040

3020

10

108

64

20

0

0

9

74

2

11 11

8.8

6.64.4

2.20

85

3

0

1310

0

500400

300200

100

12.510

7.55

2.50

1995 1996

SAN PEDRO RIVER AT CHARLESTON (Unregulated perennial)

1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

0.01 0.10 1

IN MILLIGRAMS PER LITER DISSOLVED OXYGEN,

IN CUBIC METERS PER SECOND DISCHARGE,

IN MILLIGRAMS PER LITER DISSOLVED OXYGEN,

1995 1996

GILA RIVER AT KELVIN (Regulated)

1997 1998

0.10 10 1 100

1995 1996

VERDE RIVER BELOW TANGLE CREEK (Unregulated perennial)

1997 1998

1 10 100

Figure 8. Continued. m3/s, cubic meters per second.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 45

8

64

20

10

50

2520

1510

0

1.5

3

4.5

6

7.5

108

64

20

0

1.5

3

4.5

6

7.5 7.5

6

4.53

1.50

96

3

0

1512

60

3024

1812

1512

96

30

1995 1996

SANTA CRUZ RIVER AT TUBAC (Effluent-dependent)

1997 1998

0.01 0.10 1 10

IN MILLIGRAMS PER LITER DISSOLVED OXYGEN,

IN CUBIC METERS PER SECOND DISCHARGE,

IN MILLIGRAMS PER LITER DISSOLVED OXYGEN, 1995 1996 91st AVENUE WASTEWATER-TREATMENT PLANT OUTFALL (Effluent-dependent) 1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

1 10

1995 1996

HASSAYAMPA RIVER (Effluent-dependent)

1997 1998

0.10 1 100 10

Figure 8. Continued. m3/s, cubic meters per second.
46 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

0

9060

30

150120

80

4032

2416

0

5040

3020

10

150120

9060

300

0

0

7550

25

125100

125100

7550

250

0

75

50

25

125100

0

500400

300200

100

125100

7550

250

1995 1996

SAN PEDRO RIVER AT CHARLESTON (Unregulated perennial)

1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

0.01 0.10 1

AS A PERCENTAGE OF SATURATION DISSOLVED OXYGEN,

IN CUBIC METERS PER SECOND DISCHARGE,

AS A PERCENTAGE OF SATURATION DISSOLVED OXYGEN,

1995 1996

GILA RIVER AT KELVIN (Regulated)

1997 1998

0.10 10 1 100

1995 1996

VERDE RIVER BELOW TANGLE CREEK (Unregulated perennial)

1997 1998

1 10 100

Figure 8. Continued. m3/s, cubic meters per second.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 47

0

7550

25

125100

50

2520

1510

0

1.5

3

4.5

6

7.5

125100

7550

250

0

8064

4832

16

8064

4832

160

0

80

40

200160

120

60

3024

1812

200160

12080

400

1995 1996

SANTA CRUZ RIVER AT TUBAC (Effluent-dependent)

1997 1998

0.01 0.10 1 10

AS A PERCENTAGE OF SATURATION DISSOLVED OXYGEN,

IN CUBIC METERS PER SECOND DISCHARGE,

AS A PERCENTAGE OF SATURATION DISSOLVED OXYGEN, 1995 1996 91st AVENUE WASTEWATER-TREATMENT PLANT OUTFALL (Effluent-dependent) 1997 1998 1 10

1995 1996

HASSAYAMPA RIVER (Effluent-dependent)

1997 1998

0.10 1 100 10

DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

Figure 8. Continued. m3/s, cubic meters per second.
48 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

0

80

400320

240160

80

4032

2416

0

5040

3020

10

400320

240160

800

0

0

750500

250

1,2501,000

1,2501,000

750500

2500

0

500400

300200

100

0

500400

300200

100

500400

300200

1000

1995 1996

SAN PEDRO RIVER AT CHARLESTON (Unregulated perennial)

1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

0.01 0.10 1

IN MILLIGRAMS PER LITER DISSOLVED SOLIDS,

IN CUBIC METERS PER SECOND DISCHARGE,

IN MILLIGRAMS PER LITER DISSOLVED SOLIDS,

1995 1996

GILA RIVER AT KELVIN (Regulated)

1997 1998

0.10 1 10 100

1995 1996

VERDE RIVER BELOW TANGLE CREEK (Unregulated perennial)

1997 1998

1 100 10

Figure 8. Continued. m3/s, cubic meters per second.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 49

0

500

400

300

200

100 5

0

25

20

15

10

0

1.5

3

4.5

6

7.5

500400

300200

1000

0

800

600

400

200

1,000 1,000

800

600400

2000

0

600

3,000

2,400

1,8001,200

60

30

24

1812

3,0002,400

1,8001,200

6000

1995 1996

SANTA CRUZ RIVER AT TUBAC (Effluent-dependent)

1997 1998

0.01 0.10 1 10

IN MILLIGRAMS PER LITER DISSOLVED SOLIDS,

IN MILLIGRAMS PER LITER DISSOLVED SOLIDS,

IN CUBIC METERS PER SECOND DISCHARGE, 1995 1996 91st AVENUE WASTEWATER-TREATMENT PLANT OUTFALL (Effluent-dependent) 1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

1 10

1995 1996

HASSAYAMPA RIVER (Effluent-dependent)

1997 1998

0.10 10 100 1

Figure 8. Continued. m3/s, cubic meters per second.
50 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

0

800

400

2,0001,600

1,200

80

4032

2416

0

5040

3020

10

2,0001,600

1,200800

4000

0

0

600

3,0002,400

1,8001,200

3,0002,400

1,8001,200

6000

0

800

4,0003,200

2,4001,600

0

500400

300200

100

4,0003,200

2,4001,600

8000

1995 1996

SAN PEDRO RIVER AT CHARLESTON (Unregulated perennial)

1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

0.01 0.10 1

IN MILLIGRAMS PER LITER SUSPENDED SEDIMENT,

IN CUBIC METERS PER SECOND DISCHARGE,

IN MILLIGRAMS PER LITER SUSPENDED SEDIMENT,

1995 1996

GILA RIVER AT KELVIN (Regulated)

1997 1998

0.10 10 1 100

1995 1996

VERDE RIVER BELOW TANGLE CREEK (Unregulated perennial)

1997 1998

10 1 100

Figure 8. Continued. m3/s, cubic meters per second.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 51

0

7,5006,000

4,5003,000

1,500 5

0

2520

1510

0

1.5

3

4.5

6

7.5

0

80

4032

2416

4032

2416

80

0

80,000

60,00040,000

20,000

100,000

7,5006,000

4,5003,000

1,500

100,000

80,000

60,00040,000

20,000 6

0

3024

1812

0

1995 1996

SANTA CRUZ RIVER AT TUBAC (Effluent-dependent)

1997 1998

0.01 0.10 1 10

IN MILLIGRAMS PER LITER SUSPENDED SEDIMENT,

IN CUBIC METERS PER SECOND DISCHARGE,

IN MILLIGRAMS PER LITER SUSPENDED SEDIMENT, 1995 1996 91st AVENUE WASTEWATER-TREATMENT PLANT OUTFALL (Effluent-dependent) 1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

1 10

1995 1996

HASSAYAMPA RIVER (Effluent-dependent)

1997 1998

0.10 10 100 1

Figure 8. Continued. m3/s, cubic meters per second.
52 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

0

1.501.20

0.900.60

0.30 8

0

4032

2416

0

5040

3020

10

1.501.20

0.900.60

0.300

0

0

1.60

1.200.80

0.40

2 2

1.60

1.200.80

0.400

0

0.800.64

0.480.32

0.16

0

500400

300200

100

0.800.64

0.480.32

0.160

1995 1996

SAN PEDRO RIVER AT CHARLESTON (Unregulated perennial)

1997 1998

0.01 0.10 1

IN MILLIGRAMS PER LITER TOTAL PHOSPHORUS AS PHOSPHORUS,

IN CUBIC METERS PER SECOND DISCHARGE,

IN MILLIGRAMS PER LITER TOTAL PHOSPHORUS AS PHOSPHORUS,

1995 1996

GILA RIVER AT KELVIN (Regulated)

1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/s

DISCHARGE, IN m3/s

0.10 1 10 100

1995 1996

VERDE RIVER BELOW TANGLE CREEK (Unregulated perennial)

1997 1998

1 10 100

Figure 8. Continued. m3/s, cubic meters per second.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 53

0

1.5

3

4.5

6

7.5

50

2520

1510

0

1.5

3

4.5

6

7.5

7.56

4.53

1.50

0

1.5

3

4.5

6

7.5 7.5

6

4.53

1.50

0

55

44

3322

11 6

0

3024

1812

5544

3322

110

1995 1996

SANTA CRUZ RIVER AT TUBAC (Effluent-dependent)

1997 1998

0.01 0.10 1 10

IN MILLIGRAMS PER LITER TOTAL PHOSPHORUS AS PHOSPHORUS,

IN CUBIC METERS PER SECOND DISCHARGE,

IN MILLIGRAMS PER LITER TOTAL PHOSPHORUS AS PHOSPHORUS, 1995 1996 91st AVENUE WASTEWATER-TREATMENT PLANT OUTFALL (Effluent-dependent) 1997 1998 1 10

1995 1996

HASSAYAMPA RIVER (Effluent-dependent)

1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

0.10 10 100 1

Figure 8. Continued. m3/s, cubic meters per second.
54 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

0

0.80

0.600.40

0.20

1

80

4032

2416

0

5040

3020

10

1

0

0

0

0.75

0.60

0.45

0.30

0.15

0

0

0.1050.084

0.0630.042

0.021

0.1050.084

0.0630.042

0.021

0.80

0.600.40

0.20

0.75

0.60

0.45

0.30

0.15

0

500400

300200

100

0

1995 1996

SAN PEDRO RIVER AT CHARLESTON (Unregulated perennial)

1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

0.01 0.10 1

IN MILLIGRAMS PER LITER NITRATE AS NITROGEN,

IN CUBIC METERS PER SECOND DISCHARGE,

IN MILLIGRAMS PER LITER NITRATE AS NITROGEN,

1995 1996

GILA RIVER AT KELVIN (Regulated)

1997 1998

0.10 10 1 100

1995 1996

VERDE RIVER BELOW TANGLE CREEK (Unregulated perennial)

1997 1998

1 10 100

Figure 8. Continued. m3/s, cubic meters per second.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 55

0

3.2

2.41.6

0.8

4

50

2520

1510

0

1.5

3

4.5

6

7.5

43.2

2.41.6

0.80

0

1.5

3

4.5

6

7.5 7.5

6

4.53

1.50

8

4

0

20

16

12

60

3024

1812

2016

128

40

1995 1996

SANTA CRUZ RIVER AT TUBAC (Effluent-dependent)

1997 1998

0.01 0.10 1 10

IN MILLIGRAMS PER LITER NITRATE AS NITROGEN,

IN CUBIC METERS PER SECOND DISCHARGE,

IN MILLIGRAMS PER LITER NITRATE AS NITROGEN, 1995 1996 91st AVENUE WASTEWATER-TREATMENT PLANT OUTFALL (Effluent-dependent) 1997 1998 DISCHARGE, IN m3/s DISCHARGE, IN m3/s

DISCHARGE, IN m3/s

1 10

1995 1996

HASSAYAMPA RIVER (Effluent-dependent)

1997 1998

0.10 10 100 1

Figure 8. Continued. m3/s, cubic meters per second.
56 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

0

0.15

0.12

0.09

0.06

0.03 8

0

4032

2416

0

5040

3020

10

0

0

0

0.1100.088

0.0660.044

0.022

0

0

0.0400.032

0.0240.016

0.008

0.0400.032

0.0240.016

0.008

0.15

0.12

0.09

0.06

0.03

0.1100.088

0.0660.044

0.022

0

500400

300200

100

0

1995 1996

SAN PEDRO RIVER AT CHARLESTON (Unregulated perennial)

1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

0.01 0.10 1

IN MILLIGRAMS PER LITER AMMONIA AS NITROGEN,

IN CUBIC METERS PER SECOND DISCHARGE,

IN MILLIGRAMS PER LITER AMMONIA AS NITROGEN,

1995 1996

GILA RIVER AT KELVIN (Regulated)

1997 1998

0.10 10 1 100

1995 1996

VERDE RIVER BELOW TANGLE CREEK (Unregulated perennial)

1997 1998

1 10 100

Figure 8. Continued. m3/s, cubic meters per second.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 57

0

1.5

3

4.5

6

7.5

5

0

25

20

15

10

0

1.5

3

4.5

6

7.5

7.56

4.53

1.50

8

4

0

20

16

12

2016

128

40

0

2.5

5

10

7.5

12.5

60

3024

1812

12.510

7.55

2.50

1995 1996

SANTA CRUZ RIVER AT TUBAC (Effluent-dependent)

1997 1998

0.01 0.10 1 10

IN MILLIGRAMS PER LITER AMMONIA AS NITROGEN,

IN CUBIC METERS PER SECOND DISCHARGE,

IN MILLIGRAMS PER LITER AMMONIA AS NITROGEN, 1995 1996 91st AVENUE WASTEWATER-TREATMENT PLANT OUTFALL (Effluent-dependent) 1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

1 10

1995 1996

HASSAYAMPA RIVER (Effluent-dependent)

1997 1998

0.10 10 100 1

Figure 8. Continued. m3/s, cubic meters per second.
58 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

0

3.2

2.41.6

0.8

4

80

4032

2416

0

5040

3020

10

43.2

2.41.6

0.80

0

0

3.2

2.41.6

0.8

4 4

3.2

2.41.6

0.80

0

1.51.2

0.90.6

0.3

0

500400

300200

100

1.51.2

0.90.6

0.30

1995 1996

SAN PEDRO RIVER AT CHARLESTON (Unregulated perennial)

1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

0.01 0.10 1

IN MILLIGRAMS PER LITER TOTAL NITROGEN AS NITROGEN,

IN CUBIC METERS PER SECOND DISCHARGE,

IN MILLIGRAMS PER LITER TOTAL NITROGEN AS NITROGEN,

1995 1996

GILA RIVER AT KELVIN (Regulated)

1997 1998

0.10 10 1 100

1995 1996

VERDE RIVER BELOW TANGLE CREEK (Unregulated perennial)

1997 1998

1 10 100

Figure 8. Continued. m3/s, cubic meters per second.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 59

50

2520

1510

50

2520

1510

0

1.5

3

4.5

6

7.5

2520

1510

50

50

2520

1510

2520

1510

50

50

2520

1510

60

3024

1812

2520

1510

50

1995 1996

SANTA CRUZ RIVER AT TUBAC (Effluent-dependent)

1997 1998

0.01 0.10 1 10

IN MILLIGRAMS PER LITER TOTAL NITROGEN AS NITROGEN,

IN CUBIC METERS PER SECOND DISCHARGE,

IN MILLIGRAMS PER LITER TOTAL NITROGEN AS NITROGEN, 1995 1996 91st AVENUE WASTEWATER-TREATMENT PLANT OUTFALL (Effluent-dependent) 1997 1998 DISCHARGE, IN m3/s

DISCHARGE, IN m3/sDISCHARGE, IN m3/s

1 10

1995 1996

HASSAYAMPA RIVER (Effluent-dependent)

1997 1998

0.10 10 100 1

Figure 8. Continued. m3/s, cubic meters per second.
60 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Unregulated Urban Ephemeral Reaches

There were almost no significant correlations of 

stream properties and water-chemistry constituent concentrations with streamflow at stations on unregulated 

urban ephemeral reaches (table 7). This lack of correlation reflects the high variability of the stream properties 

and water-chemistry constituent concentrations that 

occur in direct runoff. Ephemeral streamflow at these 

stations was seasonal; streamflow only occurred in 

response to precipitation from regional winter storms 

and summer thunderstorms. 

Regulated Reaches

The correlations of stream properties and waterchemistry constituent concentrations with streamflow 

at stations on regulated reaches generally were less 

significant than at stations on unregulated reaches, and 

the sign of the correlations was not as consistent 

(table 7; Gila River at Kelvin, figure 8). The number of 

cases of significant correlations of stream properties 

and water-chemistry constituent concentrations with 

streamflow was greater at stations several kilometers 

downstream from dams than at stations that were only a 

few kilometers downstream from dams. This was 

expected because the quality of the water released from 

the reservoir at any particular time should remain more 

or less constant, regardless of the discharge at which it 

is released. Any effects on water quality by streamflow 

would be dependent upon the conditions between the 

point of release and the point of sampling. Stations on 

regulated sections of the Agua Fria, Salt, and Verde 

Rivers are only a few kilometers downstream from the 

dams and have only four stream properties and waterchemistry constituent concentrations that are significantly correlated with streamflow. The Gila River at 

Winkelman and the Gila River at Kelvin, however, are 

several kilometers downstream from Coolidge Dam 

and have eight and nine stream properties and waterchemistry constituent concentrations, respectively, that 

are significantly correlated with streamflow. 

Streamflow in regulated reaches was seasonal. 

The distribution of monthly streamflow volumes was 

unimodal; the center of the modes were in February for 

the Verde River, May for the Agua Fria River, and July 

for the Gila and Salt Rivers (table 1). Comparison of 

the seasonality of streamflow upstream and downstream from large reservoirs indicates that the storage 

of water on some rivers has greatly altered the 

seasonal patterns of flow. On the Agua Fria River, the 

center of the mode of monthly streamflow volumes 

shifts from February, for the reach above the reservoir 

near Rock Springs, to May, for the reach below Waddell Dam (table 1). On the Gila River at Kelvin, the 

monthly streamflow volumes changed from a bimodal 

distribution, with modes centered in January and 

August before regulation by Coolidge Dam, to a 

unimodal distribution with the mode centered in July 

(table 1). On the Salt River, the center of the mode of 

monthly streamflow volumes shifts from March, above 

the reservoir system near Roosevelt, to July, for the 

reach below Stewart Mountain Dam (table 1). 

The seasonal variability of stream properties and 

water-chemistry constituent concentrations generally 

was not as well defined for regulated reaches as it was 

for unregulated reaches. At the Gila River at Kelvin, 

discharge was generally higher from February through 

September as a result of releases from Coolidge Dam, 

with the exception of two high flows during February 

of 1998 that resulted from runoff in the basin downstream from Coolidge dam (fig. 8). Stream temperature 

was warmer from early spring to early fall as a result of 

warmer air temperatures. Dissolved-oxygen percent 

saturation was fairly stable at about 100 percent 

through all seasons. As a result, dissolved-oxygen concentration was lower from late spring to early fall 

because of warmer stream temperatures during this 

period and because the 100-percent saturation concentration decreases with an increase in stream temperature. Concentrations of dissolved solids were also 

lower from early spring through early fall; however, 

this was a result of the higher discharges from Coolidge 

Dam during these seasons. The pH and the concentrations of suspended sediment, total phosphorus, ammonia, nitrate, and total nitrogen at the Gila River at 

Kelvin exhibited either a weak seasonality or an inconsistent seasonality from year to year. The largest variations in these stream properties and water-chemistry 

constituent concentrations occurred in the three 

samples collected from the runoff of February 1998; 

the pH decreased, and the concentrations of suspended 

sediment and the nutrients, except for ammonia, generally increased. This lack of variation, except during 

periods of runoff, indicates that seasonality has diminished for some stream properties and water-chemistry 
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 61

constituent concentrations in regulated stream reaches 

as a result of impounding runoff. There could also be a 

shift in the seasonal patterns of stream properties and 

water-chemistry constituent concentrations as a result 

of a shift in the seasonal pattern of streamflow. For 

example, concentrations of dissolved solids decrease as 

streamflow increases at the Gila River at Kelvin 

(fig. 8), so changes in the seasonality of streamflow at 

the Gila River at Kelvin probably have changed the 

seasonality of dissolved-solids concentrations. Before 

San Carlos Reservoir was in operation, concentrations 

of dissolved solids probably were high from April 

through June when streamflow typically was low, 

whereas concentrations are low during this period now 

as a result of reservoir releases.

Concentrations of dissolved solids in the Salt River 

below Stewart Mountain Dam (fig. 9) were not 

markedly seasonal; however, they did have a sinusoidal 

trend with a period of about 7 years. The troughs in the 

concentration data clearly were a function of the 

volume of water entering the reservoir system in past 

years. Because dissolved-solids concentrations were 

lower in the storm runoff and snowmelt entering the 

reservoir system than in the base flow entering the 

reservoir system, concentrations in reservoir releases 

decreased during or slightly after a year in which the 

inflow volume was large, and slowly increased until the 

next year in which the inflow volume was large. 

Dissolved solids in the reservoir systems of the Gila 

and Verde Rivers also followed this pattern; however, 

the behavior was not as clear, perhaps because their 

reservoir capacities were smaller (Cordy and others, 

1998, table 2). Patterns for other stream properties and 

water-chemistry constituent concentrations in the 

Salt River below Stewart Mountain Dam may be 

present, but are not as clearly defined. The response of 

dissolved solids in regulated streams to large inflows to 

upstream reservoirs is significant. For the reservoir 

system on the Salt River, large volume inflows are 

needed to lower the concentration of dissolved solids 

below the SMCL for drinking water (500 mg/L) in 

reservoir releases.

Effluent-Dependent Reaches

Water chemistry in effluent-dependent reaches can 

vary in time as a result of seasonally variable biological 

and chemical reaction rates, and also as a result of 

seasonally variable surface- and ground-water inflows. 

Water chemistry can vary downstream from the 

original discharge point of the effluent as a result of 

biological and chemical reactions that occur as the 

effluent flows downstream, and also as a result of 

mixing with surface- and ground-water inflows to the 

stream. The strength and sign of correlations of the 

stream properties and water-chemistry constituent 

concentrations with streamflow at stations on effluentdependent reaches varied by station (table 7 and fig. 8) 

and were related to the source of the effluent, the downstream distance from the source, and the season at the 

time of sampling.

Samples collected from the Santa Cruz River at 

Cortaro (secondary treatment) and from the 91st 

Avenue Wastewater-Treatment Plant outfall (secondary 

treatment with nitrification/denitrification) represent 

water-quality conditions at the upstream end of

effluent-dependent stream reaches during non-runoff 

conditions. Stream properties and water-chemistry 

constituent concentrations were not significantly 

correlated with streamflow at these stations, with the 

exception of dissolved-oxygen percent saturation and 

concentrations of dissolved solids and total nitrogen at 

the 91st Avenue Wastewater-Treatment Plant outfall 

and concentrations of nitrate, total ammonia and 

organic nitrogen, and total nitrogen at the Santa Cruz 

River at Cortaro (table 7). This general lack of correlation indicates that water quality is not necessarily 

related to release rates at the treatment plants, perhaps 

with the exception of total nitrogen. Stream properties 

and water-chemistry constituent concentrations for the 

91st Avenue Wastewater-Treatment Plant generally 

were seasonal; stream temperature, pH, and dissolvedoxygen percent saturation generally were highest and 

concentrations of nutrients were generally lowest from 

early spring to early fall (fig. 8). The seasonality in 

water quality could result from the higher rates of 

biological respiration and nutrient consumption during 

the warmer months of the year.

Data for the Santa Cruz River at Rio Rico are more 

representative of actual water-quality conditions in an 

effluent-dependent reach immediately downstream 

from a wastewater-treatment plant than are data for the 

Santa Cruz River at Cortaro or 91st Avenue 

Wastewater-Treatment Plant because samples at Rio 

Rico were collected during periods of runoff as well as 

during effluent-only conditions. Concentrations of 

dissolved solids and most nutrient species were negatively correlated with streamflow (table 7). The 

negative correlation for nutrients indicates that concentrations of these constituents are diluted by direct 

runoff. This process is in contrast to the increase in 

nutrient concentrations that runoff causes in 

unregulated streams.
62 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

The Santa Cruz River at Tubac is about 25 km 

downstream from the station at Rio Rico and is 

representative of an effluent-dependent stream reach 

distant to the wastewater-treatment plant. Concentrations of suspended sediment and nutrients are positively correlated with streamflow and were generally 

lowest from early spring to early fall, except when 

summer thunderstorm runoff contributed to flow 

(table 7 and fig. 8). In contrast, stream temperature, 

disolved-oxygen percent saturation, and dissolvedsolids concentration are negatively correlated with 

streamflow and were generally highest from early 

spring to early fall, except when summer thunderstorm 

runoff contributed to flow. This decrease in nutrients 

and increase in dissolved-oxygen percent saturation 

during the warmer months probably resulted from the 

growth of algae and aquatic plants in the upstream 

reach. 

The Hassayampa River is representative of stream 

reaches having base flow sustained predominantly by 

agricultural effluent from fields that are irrigated with a 

combination of ground water and treated effluent 

from the 91st Avenue Wastewater-Treatment Plant. 

When there was little irrigation from early fall to early 

spring, stream properties and water-chemistry 

constituent concentrations, except suspended sediment 

concentrations, at this station were similar to those at 

the 91st Avenue Wastewater-Treatment Plant outfall 

and at the Gila River at Buckeye Canal (fig. 8, 91st 

Avenue Wastewater-Treatment Plant outfall and 

Hassayampa River). Conversely, during the irrigation 

season from early spring to early fall, stream properties 

and water-chemistry constituent concentrations, except 

for stream temperature and ammonia concentrations, at 

this station were dissimilar to those at the 91st Avenue 

Wastewater-Treatment Plant outfall and at the Gila 

River at Buckeye Canal. In the Hassayampa River,

the stream temperature, pH, dissolved-oxygen percent 

saturation, and concentrations of dissolved oxygen, 

dissolved solids, nitrate, and total nitrogen increased 

during the irrigation season and were negatively 

0

20

40

60

80

100

120

200

1,200

400

600

800

1,000

ANNUAL COMBINED INFLOW

DISSOLVED SOLIDS AT SALT RIVER BELOW STEWART MOUNTAIN DAM

DISCHARGE, IN CUBIC METERS PER SECOND

WATER YEAR

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 1998

DISSOLVED SOLIDS, IN MILLIGRAMS PER LITER

Figure 9. Dissolved-solids concentrations at the Salt River below Stewart Mountain Dam and the annual 

combined inflow from the Salt River and Tonto Creek to the reservoirs upstream from this station, 1950–98.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 63

correlated with streamflow (table 7 and fig. 8). In 

contrast, concentrations of total phosphorus and 

ammonia decreased during the irrigation season, and 

concentrations of suspended sediment, total 

phosphorus, and total ammonia plus organic nitrogen 

were positively correlated with streamflow (table 7, fig. 

8). Concentrations of dissolved solids, nitrate, and total 

nitrogen probably increased as a result of an increase in 

the amount of irrigation wastewater that was returned 

to the Buckeye Canal and eventually flowed into the 

Hassayampa River. The decrease in ammonia and total 

phosphorus concentrations and increase in dissolvedoxygen concentrations during the warmer months 

probably resulted from the growth of algae and aquatic 

plants in the upstream reach.

Upstream Conditions

The relation of stream properties and waterchemistry constituent concentrations to upstream 

conditions, such as stream regulation, municipal and 

agricultural wastewater disposal, stream permanence, 

and land and water use, was determined by comparing 

amongst data from stations with various upstream 

conditions. The effects of impounding water in large 

reservoirs for future use and returning treated wastewater to streams after municipal or agricultural use 

were determined by comparing amongst data from 

stations on unregulated, regulated, and effluentdependent reaches. The relation of water quality to 

stream permanence reflects upstream climate and 

hydrogeologic conditions and also was determined by 

comparing amongst data from stations on unregulated 

perennial, intermittent, and urban ephemeral reaches. 

Sixteen stations representing various upstream 

conditions in the study area were selected for data 

comparison. Of the 16 stations, 4 were on unregulated 

perennial reaches, 4 were on unregulated intermittent 

or urban ephemeral reaches, 4 were on regulated 

reaches, and 4 were on effluent-dependent reaches. 

In most cases the 4 stations within each of the 4 groups 

of stations represented different upstream conditions 

within that group (table 8). Samples from each station 

were selected using specific criteria to remove temporal or streamflow biases in the water-quality data. For 

each station, only the 3 most recent samples for each of 

the 10 deciles of streamflow were selected. A variation 

of this sample selection procedure was made for unregulated intermittent and urban ephemeral stations that 

lack flow in the lower deciles of streamflow; for these 

stations, an equal number (4–7) of the most recent 

samples were selected from each non-zero decile of 

streamflow. In some cases where there were an

inadequate number of samples for a certain decile of 

streamflow, a sample from an adjacent decile of 

streamflow was used. Seasonal effects were assumed to 

be accounted for by this selection method because, as 

was shown in the previous section of this report, most 

of the seasonal trends in the stream properties and 

water-chemistry constituent concentrations were 

related to streamflow. The selection procedures 

resulted in about 30 samples for each station that were 

representative of recent conditions and were unbiased 

with respect to streamflow conditions for the station. 

Differences in the stream properties and waterchemistry constituent concentrations at the 16 stations 

were determined with the rank-sum test (Wilcoxon, 

1945) and the Tukey-Kramer honest significance 

difference (HSD; Kramer, 1956) tests using computer 

software (SPSS, Inc., 1997). Data for all stations were 

ranked to reduce the departure from a normal distribution, which is an assumption of the tests. The rank-sum 

test was performed to detect differences in the means of 

the data for two stations, for each stream property and 

water-chemistry constituent, for all possible pairs of 

stations. The Tukey-Kramer HSD test was performed 

to determine which stations had similar means of 

ranked data (Helsel and Hirsch, 1992) in the context of 

all other stations. The Tukey-Kramer HSD test requires 

that results from an analysis of variance test on data for 

all 16 stations indicate there is at least one station with 

a mean that is significantly different (p<0.05) from the 

rest; this requirement was met for every stream property and water-chemistry constituent. The rank-sum 

and Tukey-Kramer HSD tests are seemingly very 

similar, but an important distinction must be made 

about the two tests. The rank-sum test determines that 

the mean of data for one station is different from the 

mean for another station if the two means differ by a 

certain amount; this amount is based on data for only 

those two stations. The Tukey-Kramer HSD test also 

determines that the means of data for two stations are 

different on the basis of the means differing by a 

certain amount; however, this amount is based on data 

for all 16 stations, not just the 2 being compared. The 

rank-sum test is more sensitive than the Tukey-Kramer 

HSD test to differences in data between two stations; 

however, the rank-sum test does not determine this 

difference in the context of the distribution of data from 

all other stations. 
64 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Table 8. Selected upstream conditions that could affect surface-water quality at selected monitoring stations in the Central Arizona 

Basins study area

Unregulated perennial reaches

Station Land uses

Salt River near Roosevelt Rangeland

West Clear Creek Minimally developed

Verde River below Tangle Creek Mostly rangeland, small amounts of urban land use and irrigation several kilometers 

upstream

San Pedro River at Charleston Mostly rangeland, small amounts of urban land use and irrigation 

Unregulated intermittent or urban ephemeral reaches

Station Land uses

Wet Bottom Creek Rangeland

Turkey Creek Rangeland

San Pedro River at Winkelman Rangeland, and small amounts of urban land use and irrigation 

Indian Bend Wash Municipal

Regulated reaches

Station Distance downstream from regulating structure, in kilometers

Salt River below Stewart Mountain Dam 6 

Verde River below Bartlett Dam 3 

Central Arizona Project Canal near Parker 13 

Gila River at Kelvin 80 

Effluent-dependent reaches

Station Source of wastewater

Santa Cruz River at Tubac Municipal wastewater treatment plant, secondary treatment

Santa Cruz River at Cortaro Municipal wastewater treatment plant, secondary treatment

91st Avenue Wastewater-Treatment Plant outfall Municipal wastewater treatment plant, secondary treatment with 

nitrification/denitrification

Hassayampa River Treated municipal wastewater applied to agricultural fields and returned to the stream

Results from the Tukey-Kramer HSD test indicate 

that for pH, dissolved-oxygen percent saturation, and 

nutrients, stations on reaches with similar stream 

regulation, stream permanence, and disposal of wastewater conditions tend to group together; that is, the 

mean ranks generally were not significantly different at 

the p<0.05 level (table 9). This suggests that stream 

regulation, stream permanence, and wastewater 

disposal upstream from the station are important 

conditions that affect these stream properties and 

water-chemistry constituents. 

Stations on unregulated perennial reaches and 

regulated reaches generally had higher pH values than 

stations on unregulated intermittent and urban 

ephemeral, and effluent-dependent reaches. Dissolvedoxygen percent saturation generally was not significantly different among stations. The three stations on 

reaches that receive only municipal effluent, however, 

had significantly lower values than the other stations. 

Dissolved oxygen at these stations probably is low 

because of the high biological and chemical oxygen 

demand of the treated wastewater.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 65

Station

Station

X 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

pH

Verde River below Tangle Creek 1 X I

West Clear Creek 2 I X I

Verde River below Bartlett Dam 3 I X

Salt River below Stewart Mountain Dam 4 X I I I

Gila River near Kelvin 5 I X I I

Salt River near Roosevelt 6 I I X I

San Pedro River at Charleston 7 I I I X I

Central Arizona Project Canal near Parker 8 I X I I

Hassayampa River 9 I X I

San Pedro River at Winkelman 10 I I X I

Wet Bottom Creek 11 I X I

Indian Bend Wash 12 I X I I I

Turkey Creek 13 I X I I

Santa Cruz River at Tubac 14 I I X

Santa Cruz River at Cortaro 15 I I X

91st Avenue Wastewater-Treatment Plant outfall 16 X

Dissolved oxygen, percent saturation

Verde River below Bartlett Dam 1 X I I I I I I

West Clear Creek 2 I X I I I I I

Verde River below Tangle Creek 3 I I X I I I I

Central Arizona Project Canal near Parker 4 I I I X I I I I I

Salt River near Roosevelt 5 I I I I X I I I I

Salt River below Stewart Mountain Dam 6 I I I I I X I I I I

Hassayampa River 7 I I I I I I X I I I I

Gila River at Kelvin 8 I I I I X I I I

San Pedro River at Charleston 9 I I I I I X I I

Wet Bottom Creek 10 I I I I X I

Indian Bend Wash 11 I I I I X

Santa Cruz River at Tubac 12 X I

91st Avenue Wastewater-Treatment Plant outfall 13 I X

Santa Cruz River at Cortaro 14 X

Table 9. Paired-station and multiple-station comparisons of stream properties and water-chemistry constituent concentrations at

selected surface-water quality monitoring stations in the Central Arizona Basins study area

[The three most recent samples, as of 1998, for each decile of streamflow were used in this analysis. Stations are ordered by the mean rank for the

constituent from highest to lowest. I, results from the rank-sum test indicate the mean constituent level is not significantly different (p<0.05) from

that for the station with an X. Colored cells, results from the Tukey-Kramer honest significant difference test on ranked data indicate mean constituent

levels at the stations within a given column or row are not significantly different (p<0.05) from that for the station with an X. Colors indicate type of

streamflow at station: blue, unregulated perennial; green, unregulated intermittent; yellow, unregulated urban ephemeral; magenta, regulated; orange,

effluent dependent. For some constituents, stations were excluded because of an insufficient number of samples] 
66 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Dissolved solids

Hassayampa River 1 X

91st Avenue Wastewater-Treatment Plant outfall 2 X I I I

Salt River near Roosevelt 3 I X I

Gila River at Kelvin 4 I I X I

San Pedro River at Winkelman 5 I I X

Central Arizona Project Canal near Parker 6 X I I

Santa Cruz River at Cortaro 7 I X I

Salt River below Stewart Mountain Dam 8 I I X

Santa Cruz River at Tubac 9 X I I

Verde River below Tangle Creek 10 X I I

Turkey Creek 11 I I X I III

Indian Bend Wash 12 I I I X III

San Pedro River at Charleston 13 I I X I

Verde River below Bartlett Dam 14 I I I X

West Clear Creek 15 I I X I

Wet Bottom Creek 16 I X

Suspended sediment

San Pedro River at Winkelman 1 X

Hassayampa River 2 X I I

Gila River at Kelvin 3 I X I

San Pedro River at Charleston 4 I I X I I I I

Santa Cruz River at Tubac 5 I X I I I

Santa Cruz River at Cortaro 6 I I X I I I

Verde River below Tangle Creek 7 I I I X I I

Salt River near Roosevelt 8 I I I I X I

Verde River below Bartlett Dam 9 I I I X

West Clear Creek 10 X I

91st Avenue Wastewater-Treatment Plant outfall 11 I X I

Salt River below Stewart Mountain Dam 12 I X

Wet Bottom Creek 13 X

Station

Station

X 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Table 9. Paired-station and multiple-station comparisons of stream properties and water-chemistry constituent concentrations at

selected surface-water quality monitoring stations in the Central Arizona Basins study area—Continued
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 67

Total phosphorus

Santa Cruz River at Cortaro 1 X I

91st Avenue Wastewater-Treatment Plant outfall 2 I X

Hassayampa River 3 X I

Santa Cruz River at Tubac 4 I X

Turkey Creek 5 X I I

Indian Bend Wash 6 I X I I

San Pedro River at Winkelman 7 I I X I

Gila River at Kelvin 8 I I X

Verde River below Bartlett Dam 9 X I

San Pedro River at Charleston 10 I X I I

Verde River below Tangle Creek 11 I X I I

Salt River near Roosevelt 12 I I X I I I

Salt River below Stewart Mountain Dam 13 I I X

West Clear Creek 14 I X I I

Central Arizona Project Canal near Parker 15 I I X I

Wet Bottom Creek 16 I I X

Nitrate

Hassayampa River near Arlington 1 X

91st Avenue Wastewater-Treatment Plant outfall 2 X I

Santa Cruz River at Tubac 3 I X

Indian Bend Wash 4 X I

San Pedro River at Winkelman 5 I X

Turkey Creek 6 X

Santa Cruz River at Cortaro 7 X I

Central Arizona Project Canal near Parker 8 I X I I

Verde River below Bartlett Dam 9 X I I I

Gila River at Kelvin 10 I I X I I

San Pedro River at Charleston 11 I I I X I

Salt River Below Stewart Mountain Dam 12 I I I X

Wet Bottom Creek 13 X I

West Clear Creek 14 I X I

Salt River near Roosevelt 15 I X I

Verde River below Tangle Creek 16 I X

Station

Station

X 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Table 9. Paired-station and multiple-station comparisons of stream properties and water-chemistry constituent concentrations at

selected surface-water quality monitoring stations in the Central Arizona Basins study area—Continued 
68 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Ammonia

Santa Cruz River at Cortaro 1 X

91st Avenue Wastewater-Treatment Plant outfall 2 X

Santa Cruz River at Tubac 3 X I

Hassayampa River 4 I X

Indian Bend Wash 5 X

Turkey Creek 6 X

Verde River below Bartlett Dam 7 X I I I I I

San Pedro River at Charleston 8 I X I I I I I

Gila River at Kelvin 9 I I X I I I I

Central Arizona Project Canal near Parker 10 I I I X I I I

West Clear Creek 11 I I I I X I I

Salt River below Stewart Mountain Dam 12 I I I I I X I I I

Salt River near Roosevelt 13 I I I I I X I

Verde River below Tangle Creek 14 I I X I

Wet Bottom Creek 15 I I X

Total ammonia plus organic nitrogen

Santa Cruz River at Cortaro 1 X

91st Avenue Wastewater-Treatment Plant outfall 2 X

Hassayampa River 3 X I I

Turkey Creek 4 I X I I I

Santa Cruz River at Tubac 5 I I X I I

Indian Bend Wash 6 I I X I

San Pedro River at Winkelman 7 I I I X I

Gila River at Kelvin 8 I X I

Verde River below Bartlett Dam 9 I X

Salt River below Stewart Mountain Dam 10 X I I

Central Arizona Project Canal near Parker 11 I X I

San Pedro River at Charleston 12 I I X I I I

Salt River near Roosevelt 13 I X I I

Wet Bottom Creek 14 I I X I

Verde River below Tangle Creek 15 I I I X I

West Clear Creek 16 I X

Station

Station

X 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Table 9. Paired-station and multiple-station comparisons of stream properties and water-chemistry constituent concentrations at

selected surface-water quality monitoring stations in the Central Arizona Basins study area—Continued 
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 69

Total nitrogen

Santa Cruz River at Cortaro 1 X

Hassayampa River 2 X I

91st Avenue Wastewater-Treatment Plant outfall 3 I X

Santa Cruz River at Tubac 4 X I

Indian Bend Wash 5 X I I

San Pedro River at Winkelman 6 I X I

Turkey Creek 7 I I I X

Gila River at Kelvin 8 X I I

Verde River below Bartlett 9 I X I I

Central Arizona Canal near Parker 10 I I X I I

Salt River below Stewart Mountain Dam 11 I X I

San Pedro River at Charleston 12 I I I X I I

Salt River near Roosevelt 13 I X I I I

Wet Bottom Creek 14 I I X I I

West Clear Creek 15 I I X I

Verde River below Tangle Creek 16 I I I X

Station

Station

Station X 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Station

X 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Table 9. Paired-station and multiple-station comparisons of stream properties and water-chemistry constituent concentrations at

selected surface-water quality monitoring stations in the Central Arizona Basins study area—Continued

Concentrations of nutrients generally were lowest 

at stations on perennial streams, somewhat higher at 

stations on regulated reaches, and highest at stations

on effluent-dependent reaches (table 9). Nutrient 

concentrations at stations on unregulated intermittent 

and urban ephemeral reaches—Turkey Creek, the 

San Pedro River near Winkelman, and Indian Bend 

Wash—tended to group together or with concentrations 

at stations on effluent-dependent reaches. In contrast, 

nutrient concentrations for Wet Bottom Creek tended to 

group with those at stations on unregulated perennial 

reaches (table 9). Nutrient concentrations in regulated 

reaches could be different from those in unregulated 

perennial reaches because of physical processes or 

biological processes that occur in the reservoir, such as 

particle settling, nutrient consumption/production by 

aquatic life, or because reservoir outflow contains both 

base flow and runoff that were captured and mixed in 

the reservoir. Nutrient concentrations probably were 

higher at stations on unregulated intermittent and urban 

ephemeral reaches than at stations on unregulated 

perennial reaches because nutrient concentration 

tended to increase with streamflow (table 7), and the 

high-flow samples represented a larger percentage of 

the total samples for stations on unregulated intermittent and urban ephemeral reaches than for stations on 

unregulated perennial reaches. Nutrient concentrations 

were higher at stations on effluent-dependent reaches 

than at stations on unregulated reaches because the 

wastewater effluent has high concentrations of 

nutrients.

Results from the Tukey-Kramer HSD test indicate 

that dissolved-solids and suspended-sediment concentrations for stations with similar stream permanence, 

stream regulation, or return of wastewater did not 

group together. These results indicate that some other 

factor affecting water quality is more prevalent than 

these factors. Upstream basin geology and sediment 
70 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

supply are likely to be more prevalent factors; however, 

additional information concerning the upstream 

geology and abundance, availability, and size 

distribution of sediment is needed to substantiate this 

hypothesis.

The relation of surface-water quality to upstream 

conditions, such as stream regulation, municipal and 

agricultural wastewater disposal, stream permanence, 

and land and water use also was determined by performing the rank-sum test on data for two stations with 

different upstream conditions (table 9). Stream regulation was found to affect water quality as determined by 

comparison of data for stations immediately upstream 

and downstream from the large reservoirs on the Salt 

and Verde Rivers. Decreases in the dissolved solids in 

both rivers, and suspended sediment in the Salt River 

represent improvements in water quality for drinkingwater purposes, whereas increases in nitrate, total 

ammonia plus organic nitrogen, and total nitrogen 

observed in both rivers represent water-quality 

degradation. Dissolved-oxygen percent saturation was 

not affected by stream regulation, as indicated by the 

lack of significantly different data for the stations 

upstream and downstream from the reservoirs on both 

the Salt and Verde Rivers. 

Change in the quality of water from before use to 

after use reflects the effects of both land and water use. 

While the obvious effect of using stored water from 

the large reservoirs on the Agua Fria, Gila, Salt, and 

Verde Rivers for municipal and agricultural purposes 

was several kilometers of dry streambed downstream 

from the dams, there were also differences in the 

quality of the water that was returned to the stream. 

Water quality in the Salt River degraded as a result of 

municipal water use in the Phoenix metropolitan area 

as determined by comparing data from the Salt River 

below Stewart Mountain Dam and the Verde River 

below Bartlett Dam to data from the 91st Avenue 

Wastewater-Treatment Plant outfall, which constitutes 

nearly all the flow throughout most of the year at this 

point on the Salt River. The treated water returned after 

municipal use had lower values for dissolved-oxygen 

percent saturation and higher concentrations of 

nutrients than the water diverted from the streams. 

Although results from the Tukey-Kramer HSD test 

indicate that data from stations on effluent-dependent 

reaches tended to group together and were significantly 

different from data from other stations, the rank-sum 

test indicates that there were also differences amongst 

data from stations on effluent-dependent reaches. 

These differences can be attributed to the source of the 

wastewater. Comparison of data from the Santa Cruz 

River at Cortaro with data from the 91st Avenue 

Wastewater-Treatment Plant outfall indicated that 

concentrations of ammonia, total ammonia plus 

organic nitrogen, and total nitrogen were higher in 

wastewater from facilities with secondary treatment 

than in wastewater from facilities with secondary 

treatment with nitrification/denitrification, whereas 

nitrate and dissolved-oxygen percent saturation were 

lower. Total phosphorus concentrations were not significantly different for the two types of wastewater treatment. Differences in water quality between the 91st 

Avenue Wastewater-Treatment Plant outfall and the 

Hassayampa River reflected the effects of irrigating 

fields with the treated municipal effluent and with 

ground water from wells along the Buckeye Canal. The 

pH, dissolved-oxygen percent saturation, and concentrations of dissolved solids, suspended sediment, and 

nitrate were higher at the Hassayampa River than at the 

91st Avenue Wastewater-Treatment Plant outfall, 

whereas concentrations of ammonia and total ammonia 

plus organic nitrogen were lower. The increase in 

nitrate concentrations and decrease in ammonia and 

total ammonia plus organic nitrogen concentration are, 

in part, due to conversion of ammonia to nitrate and to 

the added nitrate from fertilizers in agricultural runoff. 

Results from the rank-sum test on the stream 

properties and water-chemistry constituent concentrations at the four stations on perennial reaches and at the 

four stations on unregulated intermittent and ephemeral 

reaches indicate that land use of the upstream drainage 

basin has an affect on water quality. The pH and 

dissolved-oxygen percent saturation are higher and 

concentrations of dissolved solids, suspended 

sediment, total phosphorus, nitrate, ammonia and 

organic nitrogen, and total nitrogen are lower at West 

Clear Creek than at the San Pedro River near Charleston (table 9). The stream properties and waterchemistry constituent concentrations at the Verde River 

below Tangle Creek and the Salt River near Roosevelt 

typically have values equivalent to or between those of 

West Clear Creek and those of the San Pedro River 

near Charleston (table 9). The amount of municipal and 

agricultural development in the upstream drainage 

basin follows a similar pattern—it is 
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 71

lowest at West Clear Creek, highest at the San Pedro 

River near Charleston, and in between the lowest and 

highest at the Verde River below Tangle Creek and the 

Salt River near Roosevelt (table 8). This similarity in 

the rankings of stream properties and water-chemistry 

constituent concentrations suggests that upstream land 

use affects water quality6. The four stations on 

unregulated intermittent and ephemeral reaches also 

represented different amounts of municipal or 

agricultural development (table 8). On the basis of 

results from the rank-sum test on data from the four 

stations on unregulated intermittent and ephemeral 

reaches, concentrations of dissolved solids and most 

nutrients at Wet Bottom Creek were lower than those at 

Indian Bend Wash and the San Pedro River at 

Winkelman. This difference in water quality can also 

be attributed to differences in upstream land use 

because there is municipal land use or irrigation 

upstream from the San Pedro River at Winkelman and 

Indian Bend Wash, but not upstream from Wet Bottom 

Creek. It is not clear, however, why data for Turkey 

Creek were more similar to data for the San Pedro 

River at Winkelman and Indian Bend Wash than to data 

for Wet Bottom Creek, because land use upstream from 

Turkey Creek is most similar to that for Wet Bottom 

Creek (tables 8 and 9). This discrepancy could be a 

result of differences in upstream conditions that were 

not considered in this study. 

Temporal Trends

An important aspect of monitoring surface-water 

resources is to detect changes in the quantity and quality of streamflow over time. An understanding of the 

temporal trends allows for evaluation of the effectiveness of past water-quality protection programs 

6There is a probability of less than 1 in 20 (or p<0.05) that the differences 

in a water-chemistry constituent for two stations as determined with ranksum test (table 9) are due to statistical sampling error rather than being due 

to true differences in the data. For concluding that data for a waterchemistry constituent at one station is higher (or lower) than that for three 

other stations, the probability that the differences are due to statistical 

sampling error are higher, less than 1 in 7 (p<(1– (1–0.05)3), which is 

p<0.14).

and for the determination of the need for existing or 

additional programs that will assure an adequate supply 

of good quality water in the future. 

Eight stations were selected to represent the 

temporal trends in water quality in the study area. 

The Gila River at Kelvin, the Salt River below Stewart 

Mountain Dam, and the Verde River below Bartlett 

Dam were selected because these regulated reaches 

supply most of the surface water used in the study area 

(besides that imported into the study area from the 

Colorado River). The Salt River near Roosevelt and

the Verde River below Tangle Creek were selected 

because trends in the water quality in reaches upstream 

from the reservoirs may not be apparent once streams 

enter the reservoirs. The Agua Fria River near Rock 

Springs was selected for additional spatial representation of the Central Highlands. Few human activities 

occur in the Wet Bottom Creek drainage basin, with the 

exception of livestock grazing, so it was selected as the 

best available reference station. The Gila River at 

Gillespie Dam was selected to show trends in streamwater quality at the downstream end of the study area. 

These same eight stations, along with the Verde River 

at Clarkdale, the San Pedro River at Charleston, and the 

Santa Cruz River at Cortaro Road, also were selected to 

demonstrate temporal trends in streamflow throughout 

the study area. 

Streamflow

Temporal trends in streamflow were determined on 

the basis of monthly streamflow statistics. Trends in 

high flow are represented by trends in the monthly 

maximum daily streamflow. Trends in the overall 

volume of water transported in the stream are represented by trends in the monthly mean daily streamflow. 

Trends in streamflow during low-flow conditions are 

represented by trends in the monthly 3-day low flow, 

which is the lowest average flow for 3 consecutive days 

in a month. Trends in any of these monthly statistics of 

streamflow can result from natural causes such as 

climate or vegetation change, or from human causes 

such as stream regulation, upstream diversions, or 

ground-water pumping.

Seasonal and temporal trends in the monthly flow 

statistics were demonstrated graphically (fig. 10) for 

several stations. Monthly flow statistics for each station 

were standardized on the basis of the mean and 

standard deviation of logged-flow statistics data for the 

period 1970–98. Standardizing allowed for comparison 

of the seasonal and temporal variability between

stations because the units are converted to standard 
72 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

deviations from the mean and the effects of record 

lengths are removed by standardizing to a common 

period. Years and seasons colored in blue are substantially above the average for 1970–98, and years that 

are red are substantially less than average for 1970–98. 

All stations except for the Santa Cruz River at Cortaro 

had fairly complete record for 1970–98; the small data 

gaps for this station were accommodated by the computer software (SPSS, Inc., 1997) graphing routine 

because it smoothed the data. 

Seasonal variation was greater than the long-term 

variation in the monthly flow statistics for stations on 

unregulated reaches, with the exception of the 3-day 

low flow for the Verde River near Clarkdale (fig. 10). 

Low flow did not vary much at this station. This pattern 

suggests that, compared to other stations, this station 

monitors base flow that is maintained by a large body 

of ground water and (or) that seasonal and temporal 

trends in streamflow caused by evapotranspiration 

between the ground-water source and the station were 

small as compared to that for other stations. For other 

stations on unregulated perennial streams that drain the 

Central Highlands (Salt River near Roosevelt, Verde 

River below Bartlett Dam for years 1900–38, and 

Verde River below Tangle Creek for years 1946–98 in 

figure 10) the monthly 3-day low flow, monthly mean, 

and monthly maximum daily flow tended to be higher 

from 1900 through the 1930s, lower from the 1940s 

through the 1970s, and higher again after the 1970s. 

This pattern in streamflow follows that observed for the 

mean annual precipitation for the Southwestern United 

States (Trentberth, 1991) and indicates that for streams 

in the Central Highlands, long-term trends in streamflow were driven by long-term trends in climate. 

For the San Pedro River at Charleston, which has 

unregulated perennial streamflow, seasonal variability 

in the monthly streamflow statistics was also generally 

greater than the long-term variability. In contrast to 

flows in unregulated reaches in the Central Highlands, 

the monthly mean daily flow and monthly maximum 

flows in the San Pedro River at Charleston are largest 

in the late summer rather than in the winter and early 

spring. Pool and Coes (1999) found that runoff, base 

flow, and drainage-basin precipitation declined for the 

San Pedro River at Charleston from 1936 through 1997 

and that the decline in base flow had probable causes 

including declining runoff and recharge near the river 

during June through October and increased interception of ground-water flow to the river by wells 

and phreatophytes. These declining trends also are 

apparent for the 3-day low, monthly mean, and monthly 

maximum flows shown in figure 10.

Seasonal patterns in streamflow are changed as a 

result of stream regulation (fig. 10). For example, at the 

Salt River near Roosevelt, flow was typically high from 

March through May and low to medium from June 

through February. In contrast, downstream from the 

reservoir system at the Salt River below Stewart 

Mountain Dam, flow was medium to high from March 

through October and low to medium from November 

through February. Flow was typically medium to high 

from December through March at the Gila River at 

Kelvin and the Verde River below Bartlett Dam before 

the regulation of streamflow, and flow was typically 

medium to low from April through November under 

this condition. After streamflow was regulated, however, flow tended to be low from October through 

March and medium to high from April through September.

Long-term trends in the monthly streamflow 

statistics at stations on effluent-dependent reaches were 

more extreme than those for unregulated and regulated 

reaches; in fact, the long-term variation was greater 

than the seasonal variation in the streamflow for 

stations on effluent-dependent reaches (fig. 10, Santa 

Cruz River at Cortaro, Gila River at Gillespie Dam). 

The monthly 3-day low flow increased somewhat over 

the period of record at the Santa Cruz River at Cortaro 

as a result of population growth in the Tucson metropolitan area and the corresponding increase in the 

amount of wastewater returns. Streamflow in the Gila 

River at Gillespie Dam declined from the 1930s 

through the mid 1960s, probably as a result of a lowering of the ground-water table by pumpage. Streamflow 

has increased since the mid 1960s, probably as a result 

of population growth in the Phoenix metropolitan area 

and the corresponding increase in the amount of wastewater returns to the Salt River at the 91st Avenue 

Wastewater-Treatment Plant7. Streamflow could also 

be increasing as a result of a declining trend in groundwater pumpage since the 1960s (Anning and Duet, 

1994).

7The 91st Avenue Wastewater-Treatment Plant came on line in 1958 

(E.L. Montgomery and Associates, 1986).
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 73

MONTHLY

3-DAY LOW FLOW

WET BOTTOM

CREEK

(Unregulated intermittent)

VERDE RIVER

BELOW

TANGLE CREEK

(Unregulated perennial)

VERDE RIVER

BELOW

BARTLETT DAM

(Regulated)

MONTHLY

MEAN DAILY FLOW

CENTRAL HIGHLANDS

MONTHLY

MAXIMUM DAILY FLOW

JAN.

JAN.

JAN.

Unregulated Regulated

FEB.

MAR.

APR.

MAY

JUNE

JULY

AUG.

SEPT.

OCT.

NOV.

DEC.

1940

19501960

1970

19801990

2000

1940

19501960

1970

19801990

2000

1940

19501960

1970

19801990

2000

1900

1910

19201930194019501960

1970

1980

Unregulated Regulated Unregulated Regulated

1990

2000

1900

1910

19201930194019501960

1970

19801990

2000

1900

1910

19201930194019501960

1970

19801990

2000

FEB.

MAR.

APR.

MAY

JUNE

JULY

AUG.

SEPT.

OCT.

NOV.

DEC.

FEB.

MAR.

APR.

MAY

JUNE

JULY

AUG.

SEPT.

OCT.

NOV.

DEC.

1960

1970

19801990

2000

1960

1970

19801990

2000

1960

1970

19801990

2000

EXPLANATION

DEPARTURE FROM THE MEAN OF LOGGED

MONTHLY FLOW STATISTICS—In standard

deviations. s =1 standard deviation

More than 2 s

1 s

0 s

–1 s

Less than –2 s

Figure 10. Seasonal and annual trends in logged monthly statistics of mean daily flow values at 

selected surface-water quality monitoring stations in the Central Arizona Basins study area.
74 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

MONTHLY

3-DAY LOW FLOW

SALT RIVER

NEAR ROOSEVELT

(Unregulated perennial)

JAN.

JAN.

JAN.

JAN.

FEB.

MAR.

APR.

MAY

JUNE

JULY

AUG.

SEPT.

OCT.

NOV.

DEC.

1910

19201930194019501960197019801990

2000

SALT RIVER

BELOW STEWART

MOUNTAIN DAM

(Regulated)

1930194019501960

1970

19801990

2000

1930194019501960

1970

19801990

2000

AGUA FRIA RIVER

NEAR ROCK SPRINGS

(Unregulated intermittent)

VERDE RIVER

AT CLARKDALE

(Unregulated perennial)

1960

1970

19801990

2000

1960

1970

19801990

2000

1960

1970

19801990

2000

1960

1970

19801990

2000

MONTHLY

MEAN DAILY FLOW

CENTRAL HIGHLANDS

1930194019501960

1970

19801990

2000

1960

1970

19801990

2000

1960

1970

19801990

2000

FEB.

MAR.

APR.

MAY

JUNE

JULY

AUG.

SEPT.

OCT.

NOV.

DEC.

FEB.

MAR.

APR.

MAY

JUNE

JULY

AUG.

SEPT.

OCT.

NOV.

DEC.

FEB.

MAR.

APR.

MAY

JUNE

JULY

AUG.

SEPT.

OCT.

NOV.

DEC.

1910

19201930194019501960197019801990

2000

MONTHLY

MAXIMUM DAILY FLOW

1910

19201930194019501960197019801990

2000

Figure 10. Continued.
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 75

MONTHLY

3-DAY LOW FLOW

SAN PEDRO RIVER

AT CHARLESTON

(Unregulated perennial)

JAN.

JAN.

JAN.

JAN.

FEB.

MAR.

APR.

MAY

JUNE

JULY

AUG.

SEPT.

OCT.

NOV.

DEC.

GILA RIVER

AT KELVIN

(Regulated)

SANTA CRUZ RIVER

AT CORTARO

(Effluent-dependent)

GILA RIVER

AT GILLESPIE DAM

(Effluent-dependent)

MONTHLY

MEAN DAILY FLOW

BASIN AND RANGE LOWLANDS

FEB.

MAR.

APR.

MAY

JUNE

JULY

AUG.

SEPT.

OCT.

NOV.

DEC.

FEB.

MAR.

APR.

MAY

JUNE

JULY

AUG.

SEPT.

OCT.

NOV.

DEC.

FEB.

MAR.

APR.

MAY

JUNE

JULY

AUG.

SEPT.

OCT.

NOV.

DEC.

MONTHLY

MAXIMUM DAILY FLOW

1900

1910

19201930194019501960

1970

19801990

2000

1900

1910

19201930194019501960

1970

19801990

2000

1900

1910

19201930194019501960

1970

19801990

2000

1910

19201930194019501960197019801990

2000

1910

19201930194019501960197019801990

2000

1910

19201930194019501960197019801990

2000

Unregulated Regulated Unregulated Regulated Unregulated Regulated 1970 1980 1990 2000 1970 1980 1990 2000 1970 1980 1990

2000

1930194019501960

1970

19801990

2000

1930194019501960

1970

19801990

2000

1930

194019501960

1970

19801990

2000

Figure 10. Continued.
76 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Water Quality 

Increases in water-chemistry constituent concentrations over time can indicate either that the mass load 

of that constituent delivered to the stream from natural 

or human sources has increased over time, or that 

losses of that constituent by physical or biological 

processes have decreased over time. Conversely, 

decreases in water-chemistry constituent concentration 

over time can indicate decreases in the mass load 

transported to the stream over time, or increases in 

the physical or biological processes removing the 

water-chemistry constituent over time. 

Long-term trends in water-chemistry constituents 

were determined using the seasonal Kendall test for 

trends with corrections for serial dependence (Hirsch 

and others, 1982; Hirsch and Slack, 1984). This is a 

nonparametric method suitable for detection of trends 

in seasonal data that may have a nonnormal distribution, have missing values, have censored data, or 

have serial dependence. For stream properties and 

water-chemistry constituent concentrations that are 

correlated with discharge, trends in concentration can 

be confounded by trends in streamflow at the time of 

sampling. For instance, a decrease in dissolved solids 

over time can result from an increase in streamflow at 

the time of sampling during the period of record if the 

dissolved-solids concentration is negatively correlated 

with streamflow. For cases where stream properties and 

water-chemistry constituent concentrations were correlated with streamflow (table 7, those with p<0.10), 

trends in water quality were determined by using flowadjusted concentrations (Hirsch and others, 1991) that 

were based on a LOWESS smooth (Cleveland, 1979) 

of concentration and discharge data. An increase in the 

flow-adjusted data indicates that, for a given streamflow, the concentrations of samples collected at the 

beginning of the period assessed are less than the 

concentrations of samples collected at the end of the 

period; the opposite is true for decreases in the flowadjusted data. As an indication of the magnitude of the 

trend, the Sen slope (Helsel and Hirsch, 1992, p. 266), 

which is the median of all slopes drawn between the 

possible pairs of concentration-time data points, was 

determined. 

The pH, dissolved-oxygen concentration, and dissolved-oxygen percent saturation of samples generally 

increased or had no trend over the period of sampling at 

the eight stations (table 10). Stream pH increased in the 

Salt River below Stewart Mountain Dam,

the Verde River below Bartlett Dam, the Gila River at 

Kelvin, and the Gila River at Gillespie Dam, but had no 

trend at the Salt River near Roosevelt, Wet Bottom 

Creek, the Verde River below Tangle Creek, and the 

Agua Fria River near Rock Springs. Temporal trends in 

pH may cause trends in concentrations of some 

constituents such as arsenic or other trace elements that 

have solubilities related to pH. Concentrations of 

dissolved oxygen increased at the Salt River near 

Roosevelt, the Salt River below Stewart Mountain 

Dam, and the Verde River below Tangle Creek. 

Concentrations of dissolved oxygen had no trend at the 

other stations. The dissolved-oxygen percent saturation 

increased at the Verde River below Tangle Creek and 

the Gila River at Gillespie Dam.

Concentrations of dissolved solids decreased at the 

Salt River below Stewart Mountain Dam (fig. 9) and 

the Verde River below Bartlett Dam, increased at the 

Gila River at Kelvin, and had no trend at the other five 

stations (table 10). The decrease in dissolved-solids 

concentrations in the Salt and Verde Rivers is an important improvement in water quality because these rivers 

are used as a drinking-water source for the Phoenix 

metropolitan area and concentrations of dissolved 

solids in the Salt River are typically high (fig. 6), often 

exceeding the SMCL. The decreases in concentration 

could have resulted either from an increase in the 

volume of runoff entering the reservoirs or from a 

decrease in the load of dissolved solids being delivered 

to the river. The former explanation suggests that these 

trends were driven by changes in climate. This is the 

more likely explanation because the monthly mean 

daily streamflow increases over the period of sampling 

at these stations (fig. 10) and because the flow-adjusted 

concentrations do not change over the period of record 

in the Salt River near Roosevelt or in the Verde River 

below Tangle Creek. The increase in concentrations at 

the Gila River at Kelvin suggests that there is an 

increase in the load of dissolved solids delivered to the 

river from one or more natural or human sources.

 Concentrations of suspended sediment at six of the 

stations had no trend, but at the Salt River below Stewart Mountain Dam and the Gila River at Gillespie Dam 

concentrations decreased over the period of sampling 

(table 10). Concentrations probably decreased at the 

Salt River below Stewart Mountain Dam as a result of 

impounding sediments in the series of four reservoirs 

upstream from the station. This is not the cause for the 

decrease observed in concentrations at the Gila River at 

Gillespie Dam, however, because this dam is a diversion dam with a low storage capacity. 
Relation of Stream Properties and Water-Chemistry Constituent Concentrations to Natural and Human Factors 77

Table 10. Monotonic trends in stream properties and water-chemistry constituent concentrations at selected surface-water quality 

monitoring stations in the Central Arizona Basins study area, through 1998

[Data in each cell are beginning and ending year for analysis, Sen slope of the trend, and statistical significance that the slope is not equal to zero as 

determined by using the seasonal Kendall’s test with correction for serial correlation (increasing tends: +++, p<0.01; ++, p<0.05; +, P<0.10; decreasing 

trends: ---, p<0.01; --, p<0.05; -, p<0.10; 0, trend is not significant). Purple shading, increasing trend; lavender shading, no trend; blue shading, decreasing 

trend. *, trend analysis performed on flow-adjusted concentrations] 

Station pH

Dissolved-oxygen

Dissolved 

solids

Suspended 

sediment

Total 

phosphorus Nitrate Ammonia

Total 

nitrogen

ConcentrationPercent 

saturation

Central Highlands

Salt River near Roosevelt 

(unregulated, perennial)

76-98

0.005*

0

82-98

0.041*

++

82-98

<0.0001

0

76-98

1.11*

0

78-98

1.10*

0

82-98

-0.0011*

0

76-98

<0.0001

0

84-98

<0.0001

0

82-98

-0.024*

---

Salt River below Stewart 

Mountain Dam (regulated)

50-96

0.015

+++

77-92

0.163

++

82-92

-0.343*

0

50-92

-0.349*

--

76-92

-0.57

--

74-92

-0.0007*

-

70-92

<0.0001

0

77-92

-0.0019

--

74-92

-0.005

0

Wet Bottom Creek 

(unregulated, intermittent)

68-96

0.001

0

79-96

0.013*

0

82-96

-0.333

0

69-96

-0.396*

0

79-96

-0.04*

0

77-96

-0.0002*

0

71-96

<0.0001

0

80-96

-0.0019*

--

80-96

-0.025

---

Verde River below 

Tangle Creek 

(unregulated, perennial)

80-97

0.003

0

80-97

0.043*

++

82-97

0.207*

++

80-97

0.705*

0

81-97

0.10*

0

80-97

-0.0009*

0

80-97

-0.0020*

--

84-97

-0.0012*

---

80-97

-0.021*

---

Verde River below 

Bartlett Dam 

(regulated)

50-92

0.011*

+++

77-92

0.017

0

82-92

-0.0536

0

50-92

-2.35*

---

75-92

-0.14

0

74-92

<0.0001

0

70-92

<-0.0001

--

77-92

-0.0017*

-

74-92

0.001*

0

Agua Fria River near 

Rock Springs 

(unregulated, intermittent)

82-95

-0.002*

0

82-95

-0.033*

0

82-95

-0.620

0

82-95

0.782*

0

82-95

-0.08

0

82-95

-0.0013*

0

82-95

-0.0029*

0

82-95

<0.0001

0

82-95

-0.020*

--

Basin and Range Lowlands

Gila River at Kelvin 

(regulated)

50-81

0.014*

+++

Record 

not long 

enough for 

analysis

Record 

not long 

enough for 

analysis

50-77

11.4*

+++

75-98

-19.7*

0

70-77

-0.0355*

0

Record 

not long 

enough for 

analysis

Record 

not long 

enough for 

analysis

Record 

not long 

enough for 

analysis

Gila River at 

Gillespie Dam 

(effluent-dependent)

59-98

0.006*

+++

77-98

0.020

0

82-98

2.00

++

59-98

-6.35*

0

75-98

-2.27

--

74-98

-0.0546*

---

74-98

-0.0550*

--

77-98

-0.0490*

--

78-98

-0.214*

---

Concentrations of nutrients at the eight stations 

either decreased over the period of sampling or had no 

trend (table 10). Concentrations of total phosphorus 

decreased over the period of sampling at the Salt River 

below Stewart Mountain Dam and the Gila River at 

Gillespie Dam. The decrease in total phosphorus at the 

Gila River at Gillespie Dam could have resulted from 

the decrease in suspended sediment because phosphorus sorbs to sediment. Concentrations of nitrate 

decreased at the Verde River below Tangle Creek, the 

Verde River below Bartlett Dam, and the Gila River at 

Gillespie Dam; concentrations of ammonia decreased 

at these same stations and also at the Salt River below 

Stewart Mountain Dam and Wet Bottom Creek. 

Concentrations of total nitrogen decreased over the 

period of sampling at the Salt River near Roosevelt, 

Wet Bottom Creek, the Verde River below Tangle 

Creek, the Agua Fria River near Rock Springs, and 

the Gila River at Gillespie Dam. The decreases in 

concentrations of nutrients were largest at the Gila 

River at Gillespie Dam, as indicated by the Sen slope 

(table 10), and indicate considerable improvement in 

water quality, not only because there is more water in 

the stream (fig. 10), but because loads of nitrogen to the 

stream from natural and human sources also have 

decreased and (or) nutrient uptake by aquatic biota and 

riparian vegetation has increased. 
78 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

On the basis of results from the stations used in 

this study, temporal trends in streamflow and in stream 

properties and water-chemistry constituent concentrations commonly occur over the period of a decade 

or longer in streams in the study area. Temporal trends 

in streamflow and concentrations of dissolved solids 

at stations on unregulated and regulated reaches in the 

Central Highlands appear to be caused by trends in

climate. In contrast, temporal trends in water management and in climate probably have caused the temporal 

trends in streamflow at stations on effluent-dependent 

reaches in the Basin and Range Lowlands and trends in 

concentrations of dissolved solids or nutrients in the 

Gila River at Gillespie Dam. Concentrations of 

dissolved solids or nutrients generally have decreased 

over time at most stations. In some cases the decrease 

has been quite substantial. Dissolved oxygen and pH 

generally have remained the same or increased. 

Temporal trends in pH may cause trends in concentrations of some constituents such as arsenic or other trace 

elements that have solubilities related to pH.

RELATION OF STREAM LOADS TO UPSTREAM 

CONDITIONS

A comparison of stream loads of water-chemistry 

constituents at different locations along the stream and 

a comparison of the stream loads to upstream inputs of 

the constituent from natural and anthropogenic sources 

can provide information about the relative importance 

of different sources and also can provide insight to the 

fate of the water-chemistry constituent (Likens and 

Bormann, 1995). To gain this information and insight, 

stream loads and upstream inputs of dissolved solids, 

nitrogen, and phosphorous were estimated at several 

surface-water quality monitoring stations in the study 

area. The methods used in this study provide quantities 

of mass input and output from a basin; however, the 

input and output values have a large amount of 

uncertainty, and therefore, the results of this analysis 

should be considered qualitative and exploratory.

Transport of water-chemistry constituents out of 

a basin in streamflow is controlled partly by the

portion of the precipitation that runs off as streamflow. 

The percentage of the annual precipitation volume 

that runs off as streamflow is low for stations in the 

study area and ranges from 1 to 15 percent (table 11); 

the percentage is larger for stations in the Central 

Highlands than for stations in the Basin and Range 

Lowlands. Annual streamflow in 1996 and 1997 was 

less than average at most stations because of drought 

conditions (Baynham and Phillips, 1996). Annual 

streamflow in 1996 at the Hassayampa River, however, 

was about average, and streamflow in 1996 at the 

Gila River at Kelvin was above average. Annual 

streamflow in 1998 was less than average at the 

Agua Fria River near Rock Springs, the San Pedro 

River near Charleston, and the Gila River at Gillespie 

Dam; about average at the Salt River near Roosevelt, 

the Gila River at Kelvin, and the Hassayampa River; 

and above average at West Clear Creek and the 

Verde River below Tangle Creek. 

Annual stream loads of dissolved solids, total phosphorus, nitrate, total ammonia plus organic nitrogen, 

and total nitrogen were estimated for selected sites for 

the 1996–98 water years (table 12). The stream loads

were estimated using multiple regression; details of the 

estimation methods are explained in appendix 2. The 

annual load of total nitrogen was estimated as the sum 

of the annual loads of nitrate and total ammonia plus 

organic nitrogen. For the Santa Cruz River at Cortaro 

and the 91st Avenue Wastewater-Treatment Plant 

outfall, annual loads were estimated only for years 

when water-quality samples were collected and 

exclude loads carried in storm runoff. For the few days 

with direct runoff, the mean daily flow was replaced 

with the median of the mean daily flows for that month. 

Annual yields of the water-chemistry constituents 

facilitate comparison of loads amongst basins and were 

calculated as the annual load divided by the basin area. 

Annual yields were not calculated for canals or

effluent-dependent reaches. 

Dissolved Solids

Estimates of annual stream loads of dissolved 

solids at stations in the study area from 1996–98 

ranged from 1.9 million kilograms per year at the 

Agua Fria River near Rock Springs in 1996 to 980 million kilograms per year at the Central Arizona Project 

Canal near Parker in 1998 (table 12). Estimates of 

annual stream loads of dissolved solids are relatively 

precise compared to annual stream loads of nutrients 

and have a range in standard error of prediction from 

1 to 9 percent (table 12).
Relation of Stream Loads to Upstream Conditions 79

Table 11. Annual streamflow for water years 1996–98 and mean annual streamflow for the period of record at selected surface-water 

quality monitoring stations in the Central Arizona Basins study area

[---, not computed]

Station name

Annual streamflow, in cubic meters per second 

(annual streamflow as a percentage of the mean 

for the period of record1)

Mean annual streamflow 

for the period of record, 

as a percentage of average 

annual precipitation 

volume1 1996 1997 1998

Central Highlands

Salt River near Roosevelt 6.68

(26)

15.5

(60)

26.1

(100)

13

West Clear Creek .530

(28)

1.20

(63)

2.59

(136)

15

Verde River below Tangle Creek 6.40

(38)

9.32

(56)

21.6

(129)

7

Agua Fria River near Rock Springs .173

(7)

.377

(14)

1.87

(70)

7

Basin and Range Lowlands

Central Arizona Project Canal near Parker 30.5

(---)

44.3

(---)

53.1

(---)

---

Gila River at Kelvin3 18.0

(133)

11.8

(82)

13.0

(96)

2.1

San Pedro River at Charleston .541

(41)

.311

(23)

.614

(46)

3.7

Santa Cruz River at Tubac .493

(78)

.442

(70)

---

(--)

1.0

Santa Cruz River at Cortaro2 1.51

(---)

---

(---)

---

(---)

---

91st Avenue Wastewater-Treatment Plant outfall2 4.30

(---)

3.74

(---)

---

(---)

---

Gila River at Buckeye Canal ---

(---)

5.14

(---)

5.52

(---)

---

Hassayampa River 1.82

(101)

1.18

(65)

1.72

(95)

---

Gila River at Gillespie Dam3 4.19

(27)

4.45

(29)

6.31

(41)

1.1

1Mean annual streamflow from table 1; average annual precipitation volume estimated using drainage basin areas and mean annual precipitation data listed in Garrett and 

Gellenbeck, 1991. 2Streamflow excludes runoff.

3Includes drainage area above Coolidge Dam.
80 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Table 12. Annual stream load and yield of selected water-chemistry constituents at selected surface-water quality monitoring stations in

the Central Arizona Basins study area, water years 1996–98

[R2, coefficient of determination for the concentration regression equation; ---, not computed]

Station name

Number 

of 

samples R2

Annual load, 

in millions of kilograms

Annual yield, 

in kilograms per square kilometer

Standard error of 

prediction, 

in percent

1996 1997 1998 1996 1997 1998 1996 1997 1998

Dissolved solids

Central Highlands

Salt River near Roosevelt 219 93 330 420 480 30,000 38,000 43,000 3 3 3

West Clear Creek 46 94 3.1 4.8 8.0 5,000 7,700 13,000 2 2 3

Verde River below Tangle Creek 159 92 71 85 125 4,700 5,600 8,000 1 2 2

Agua Fria River near Rock Springs 162 31 1.9 3.7 18 670 1,300 6,300 4 6 5

Basin and Range Lowlands

Central Arizona Project Canal 

near Parker

41 (

1) 970 820 980 (2) (2) (2) 4 4 4

Gila River at Kelvin 406 52 370 270 290 8,000 5,800 6,200 3 3 3

San Pedro River at Charleston 83 33 4.0 2.6 4.6 1,300 810 1,400 5 4 5

Santa Cruz River at Tubac 27 51 6.1 5.4 --- 2,000 1,700 --- 5 5 ---

Santa Cruz River at Cortaro3 12 49 29 --- --- (

2) --- --- 5 --- ---

91st Avenue Wastewater-Treatment 

Plant outfall3

24 51 100 110 --- (

2) (2) --- 4 4 ---

Gila River at Buckeye Canal 36 (

1) --- 210 230 --- (2) (2) --- 2 2

Hassayampa River 48 67 75 61 74 20,000 16,000 19,000 6 6 6

Gila River at Gillespie Dam 380 23 350 440 440 2,700 3,400 3,400 8 9 9

Total phosphorus

Central Highlands

Salt River near Roosevelt 260 33 .016 .027 .070 1.4 2.4 6.3 24 26 36

West Clear Creek 39 55 .001 4.002 (

4) .41 43.8 (4) 30 59 209

Verde River below Tangle Creek 197 31 .009 .025 .17 .62 1.7 11 13 23 35

Agua Fria River near Rock Springs 166 48 .002 .003 .010 .57 1.2 3.6 40 46 36

See footnotes at end of table.
Relation of Stream Loads to Upstream Conditions 81

Table 12. Annual stream load and yield of selected water-chemistry constituents at selected surface-water quality monitoring stations in the 

Central Arizona Basins study area, water years 1996–98—Continued

Station name

Number 

of 

samples R2

Annual load, 

in millions of kilograms

Annual yield, 

in kilograms per square kilometer

Standard error of 

prediction, in percent

1996 1997 1998 1996 1997 1998 1996 1997 1998

Total phosphorus—continued

Basin and Range Lowlands

Central Arizona Project Canal near 

Parker 

41 (

1) .021 .018 .022 (2) (2) (2) 7 7 7

Gila River at Kelvin 92 31 .25 .12 .15 5.2 2.5 3.2 31 31 33

San Pedro River at Charleston 86 42 .006 4.002 4.005 1.7 4.59 41.6 47 56 52

Santa Cruz River at Tubac 26 87 .023 .058 --- 7.4 18 --- 12 15 ---

Santa Cruz River at Cortaro3 12 47 .21 --- --- (

2) --- --- 15 --- ---

91st Avenue Wastewater-Treatment 

Plant outfall3

23 (

1) .55 .48 --- (2) (2) --- 5 5 ---

Gila River at Buckeye Canal 35 47 --- .45 .46 --- (

2) (2) --- 3 6

Hassayampa River 48 50 .19 .10 .14 50 26 38 12 14 11

Gila River at Gillespie Dam 250 29 .21 .28 .27 1.6 2.1 2.1 10 12 12

Total ammonia plus organic nitrogen

Central Highlands

Salt River near Roosevelt 260 33 .074 .11 .18 6.6 9.2 16 16 17 21

West Clear Creek 39 48 .002 .007 (

4) 2.9 11 (4) 19 26 109

Verde River below Tangle Creek 197 46 .029 .040 (

4) 1.9 2.6 (4) 12 15 70

Agua Fria River near Rock Springs 166 38 .001 .002 .011 .47 .77 3.8 37 55 36

Basin and Range Lowlands

Central Arizona Project Canal near 

Parker 

40 (

1) .47 .40 .47 (2) (2) (2) 23 23 23

Gila River at Kelvin 85 36 .60 .31 .38 13 6.6 8.2 21 21 23

San Pedro River at Charleston 86 24 (4) (

4) (4) (4) (4) (4) 165 178 245

Santa Cruz River at Tubac 26 88 .046 .18 --- 15 57 --- 22 27 ---

Santa Cruz River at Cortaro3 12 49 1.4 --- --- (

2) --- --- 4 --- ---

91st Avenue Wastewater-Treatment 

Plant outfall3

24 68 1.4 .59 --- (

2) (2) --- 16 11 ---

Gila River at Buckeye Canal 36 87 --- .61 .24 --- (

2) (2) --- 6 11

Hassayampa River 48 63 .43 .12 .11 110 32 28 21 11 13

Gila River at Gillespie Dam 244 6 .40 .60 .59 3.1 4.6 4.5 11 15 15

Nitrate

Central Highlands

Salt River near Roosevelt 230 17 .006 .010 .023 .53 .89 2.1 22 24 31

West Clear Creek 44 (

1) .001 .001 .003 .81 1.8 4.7 9 9 9

Verde River below Tangle Creek 85 (

1) .004 .006 .015 .29 .41 .96 8 8 8

Agua Fria River near Rock Springs 165 (

1) .001 .001 .002 .06 .13 .67 12 12 12

See footnotes at end of table.
82 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Nitrate—continued

Basin and Range Lowlands

Central Arizona Project Canal near 

Parker

40 (

1) .24 .20 .24 (2) (2) (2) 15 15 15

Gila River at Kelvin 194 28 .12 .057 .065 2.5 1.2 1.4 30 30 31

San Pedro River at Charleston 74 37 .004 4.001 4.004 1.4 4.44 41.2 48 55 51

Santa Cruz River at Tubac 27 73 .034 .055 --- 11 18 --- 12 13 ---

Santa Cruz River at Cortaro3 12 34 .020 --- --- (

2) --- --- 29 --- ---

91st Avenue Wastewater-Treatment 

Plant outfall3

24 23 .35 .36 --- (

2) (2) --- 17 15 ---

Gila River at Buckeye Canal 36 (

1) --- .70 .76 --- (2) (2) --- 9 9

Hassayampa River 48 63 .40 .31 .40 100 81 100 5 5 5

Gila River at Gillespie Dam 190 33 1.3 1.4 1.4 9.8 11 11 16 16 16

Total nitrogen (estimated as the sum of total ammonia plus organic nitrogen and nitrate)

Central Highlands

Salt River near Roosevelt --- --- .080 .12 .20 7.1 10 18 15 16 19

West Clear Creek --- --- .002 .008 (

4) 3.70 13 (4) 17 23 109

Verde River below Tangle Creek --- --- .033 .046 4.79 2.2 3.0 452 12 14 69

Agua Fria River near Rock Springs --- --- .002 .003 .013 .53 .89 4.5 34 49 32

Basin and Range Lowlands

Central Arizona Project Canal near 

Parker

--- --- .71 .60 .71 (

2) (2) (2) 20 20 20

Gila River at Kelvin --- --- .72 .36 .45 15 7.8 9.6 23 22 24

San Pedro River at Charleston --- --- (

4) (4) (4) (4) (4) (4) 160 170 244

Santa Cruz River at Tubac --- --- .079 .23 --- 25 75 --- 17 23 ---

Santa Cruz River at Cortaro --- --- 1.5 --- --- b --- --- 4 --- ---

91st Avenue Wastewater-Treatment 

Plant outfall3

--- --- 1.7 .96 --- (

2) (2) --- 16 12 ---

Gila River at Buckeye Canal --- --- --- 1.3 1.0 --- (

2) (2) --- 7 9

Hassayampa River --- --- .83 .43 .50 220 110 130 13 6 7

Gila River at Gillespie Dam --- --- 1.7 2.0 2.0 13 16 16 13 16 16

1Stream load and yield estimates are based on the mean concentration rather than on a regression equation. 2The annual yield was not estimated for stations on canals or on reaches in which the stream load excludes transport in runoff. 3Stream load and yield exclude transport in runoff.

4The 95-percent confidence interval for the stream load and yield is larger than the estimate of the stream load and yield. In cases where the 95-percent confidence interval is 

excessively large, the stream load and yield are not tabulated. 

Table 12. Annual stream load and yield of selected water-chemistry constituents at selected surface-water quality monitoring stations in the 

Central Arizona Basins study area, water years 1996–98—Continued

Station name

Number 

of 

samples R2

Annual load, 

in millions of kilograms

Annual yield, 

in kilograms per square kilometer

Standard error of 

prediction, in percent

1996 1997 1998 1996 1997 1998 1996 1997 1998
Relation of Stream Loads to Upstream Conditions 83

Dissolved solids in streamflow may originate 

from precipitation, geologic sources such as evaporite 

deposits and weathered rocks, or human activities 

such as the release of wastewater to land surfaces or 

water bodies, or the application of fertilizers to 

cultivated lands. Precipitation scavenges ions from 

the atmosphere as it falls and contains some dissolved 

solids (fig. 7), and the runoff flowing overland or 

through the subsurface can dissolve salts on its path

to the stream. As a result, annual stream loads of 

dissolved solids are larger for wet years than for dry 

years. For stations draining the Central Highlands, 

the annual stream load of dissolved solids generally 

was higher in 1998, a wet year with a high annual 

streamflow, than in 1996, a dry year with a low 

mean annual streamflow (tables 11 and 12). The 

relative importance of precipitation as a source of 

dissolved solids can be assessed by comparing the 

annual input of dissolved solids from precipitation 

for a basin to the stream load of dissolved solids

transported out of the basin; the remainder must 

come from other sources such as fertilizers, household 

cleansers, surface or subsurface geologic deposits, 

or other sources. The annual input of dissolved 

solids from precipitation typically represents less 

than half of the total stream load observed at the 

Salt River near Roosevelt, West Clear Creek, the 

Verde River below Tangle Creek, the Agua Fria River 

near Rock Springs, and the Gila River at Gillespie 

Dam (table 13). At these stations, other sources, such 

as geologic formations, must be more significant contributors to the annual stream load; in contrast, nearly 

all, 97 percent, of the stream load of dissolved solids 

could be attributed to precipitation at the San Pedro 

River near Charleston (table 13). In these comparisons 

of input mass to output mass, an important consideration is that the actual material input can be different 

from that in the stream load. For instance, most of the 

dissolved solids input to the basin for the San Pedro 

River at Charleston is from precipitation; however, the 

dissolved solids could remain in the basin near the land 

surface where the precipitation falls, and the mass of 

dissolved solids observed in the stream load could 

come mostly from other sources, such as subsurface 

geologic formations.

At the Gila River at Gillespie Dam, dissolved 

solids most likely originate from subsurface geologic 

sources and human sources. Stream loads of dissolved 

solids nearly doubled between the 91st Avenue Wastewater-Treatment Plant outfall and the Gila River at 

Buckeye Canal, and doubled again between the

Gila River at Buckeye Canal and the Gila River at 

Gillespie Dam. This increase can be attributed to both 

saline ground-water inflow and irrigation return flows. 

The inflow of saline ground water to the Gila River is 

not well documented; however, it is inferred from 

Table 13. Average annual input of dissolved solids from 

precipitation to drainage basins of selected surface-water quality 

monitoring stations in the Central Arizona Basins study area

Station

 Annual input of 

dissolved solids from 

precipitation1, 

1980–96, in millions of 

kilograms per year

Annual input of 

dissolved solids from 

precipitation, 1980–96, 

as a percentage of the 

average annual stream 

load of dissolved 

solids for water years 

1996–98

Central Highlands

Salt River near 

Roosevelt

17 4.1

West Clear Creek 1.0 19

Verde River 

below Tangle 

Creek

19 20

Agua Fria River 

near Rock 

Springs

3.2 40

Basin and Range Lowlands

San Pedro River 

at Charleston

3.6 97

Santa Cruz River 

at Tubac

3.8 66

Gila River at 

Gillespie Dam2

90 22

1The annual input of dissolved solids in precipitation is estimated on the basis of 

drainage area and mean annual precipitation (Garrett and Gellenbeck, 1991) and the 

mean concentration of dissolved solids in precipitation at the three atmospheric 

deposition stations for 1980–96 (fig. 7; National Atmospheric Deposition Program, 

1998b). 2Includes drainage area above Coolidge Dam.
84 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

several lines of evidence. The effluent-dependent reach 

on the Salt and Gila Rivers between the 91st Avenue 

Wastewater-Treatment Plant outfall and the Buckeye 

Canal gained streamflow. Some of this gain came 

from the Gila River upstream from the mouth of the 

Salt River. This reach of the Gila was sampled during 

low-flow conditions in March 1996 and had a 

dissolved-solids concentration of 7,210 mg/L at a 

streamflow of about 0.5 m3/s (Tadayon and others, 

1998, p. 342). In the area along the Gila River from 

the mouth of the Salt River to Gillespie Dam, ground 

water generally was shallow (Brown and Pool, 1989) 

and had a similar salinity to the stream sample just 

mentioned (Kister and Hardt, 1966; Kister, 1974; 

Brown and Pool, 1989). High salinity of the ground 

water upgradient from the confluence of the Gila and 

Salt Rivers is thought to be associated with subsurface 

evaporite deposits (Kister and Hardt, 1966); however,

it also could have resulted, in part, from recharge of 

saline irrigation wastewater. Likewise, salinity in the 

Gila River could also result from surface return of 

irrigation wastewater. Although irrigation return flows 

are known to enter this reach of the Gila, the fact that 

the dissolved-solids concentrations and stream loads 

increased from the 91st Avenue Wastewater-Treatment 

Plant to the Gila River at Buckeye Canal and from 

the Gila River at Buckeye Canal to the Gila River at 

Gillespie Dam, during the non-irrigation season as

well as during other times of the year, suggests that 

saline ground-water inflows were a major source of 

the dissolved solids.

Dissolved solids transported in streamflow 

from the Central Highlands and from the Central 

Arizona Project Canal were retained in the Basin 

and Range Lowlands. For example, during 1997 

about 1,600 million kilograms of dissolved solids 

was transported into the Basin and Range Lowlands 

from the Gila, Salt, and Verde Rivers, and the Central 

Arizona Project Canal; however, only 440 million 

kilograms was transported out of the study area in the 

Gila River (fig. 11 and table 12). The difference, about 

1,200 million kilograms of dissolved solids per year, 

was most likely stored in soils, unsaturated zones, and 

aquifers in agricultural and urban areas as a result of 

irrigating crops and urban vegetation. Much of the 

applied irrigation water is evapotranspired as pure 

water, and the remainder is a highly mineralized water 

that either becomes surface runoff or percolates 

through the soils and unsaturated zone and recharges 

the aquifer. As a result of the evaporation and because 

the dissolved solids generally remain in solution, the 

percolating water is much more concentrated than 

the original irrigation water, and the water in the soil, 

the unsaturated zone, and the aquifer becomes more 

saline. This process was evident in an agricultural

area west of Phoenix, where high concentrations of

dissolved solids in shallow ground water are believed 

to be the result of irrigation seepage (Edmonds and 

Gellenbeck, 2002).

Nutrients

 Annual stream loads of total phosphorus, total 

ammonia plus organic nitrogen, nitrate, and total 

nitrogen were estimated for selected sites for water 

years 1996–98 (table 12 and fig. 12). The standard 

error of prediction of stream load estimates for the 

nutrients typically was high and ranged from 3 to 245 

percent. For several cases, the standard error of prediction was much greater than 50 percent and precluded 

using results for certain years. The standard error of 

prediction was particularly high for nutrient stream 

loads for the San Pedro River at Charleston as a result 

of the large variability in concentration in higher 

streamflows and as a result of the small number of 

samples that were collected during high flows. Stream 

loads of total phosphorus, total ammonia plus organic 

nitrogen, nitrate, and total nitrogen generally were 

much greater in years with higher annual streamflow 

than in years with lower annual streamflow. 

Along the lower Salt River and the Gila River 

west of Phoenix, stream loads of phosphorus tended

to decrease downstream, whereas stream loads of 

nitrogen tended to increase (table 12 and fig. 12). 

For 1997, stream loads of nitrogen increased from 

0.96 million kilograms per year at the 91st Avenue 

Wastewater-Treatment Plant outfall to 1.3 million 

kilograms per year at the Gila River at Buckeye 

Canal and to 2.0 million kilograms per year at the

Gila River at Gillespie Dam. The increase probably 

resulted from additional inputs of fertilizer in

downstream areas. For 1997, the stream load of

phosphorus decreased from 0.48 million kilograms 

per year at the 91st Avenue Wastewater-Treatment 

Plant outfall to 0.45 million kilograms per year at the 

Gila River at Buckeye Canal and to 0.28 million 

kilograms per year at the Gila River at Gillespie Dam.
Relation of Stream Loads to Upstream Conditions 85

1, 2

0

2

GILA

PINAL

SANTA

CRUZ

NAVAJO

GREENLEE

GRAHAM

PIMA

YAVAPAI

APACHE

GREENLEE

GRAHAM

COCHISE

COCONINO

MARICOPA

GILA

Sedona

Cottonwood

Saint

David

Nogales

Gila

Bend

PHOENIX

Eloy

Miami Globe

Payson McNary

Flagstaff

Winkelman

Kelvin

Green Valley

Cananea

Sierra

Vista

Camp Verde

Clarkdale

Dewey

Rock

Springs

Pinetop-Lakeside

Stanfield

Buckeye

Chandler

Roosevelt

Laveen

TUCSON

Prescott

ARIZONA

MEXICO

Apache

Lake

C r Cr

Wash

Cr

Hassayampa River

New

R

Verde

River

West Clear Cr

Cave Cr

Syc amoreCr

Tonto Cr

C herry Creek

Cibeque Cr

Canyon Creek

Carri zo

Cr

River Whitee

Black

Rive

B

r

onito

Gila

River

Santa

Cruz

Altar Wash

Waterman Wash Santa Rosa

San

Pedro

River

River

Santa Cruz Wash Salt

River

Theodore

Roosevelt

Lake

Lake Pleasant

Painted

Rock

Reservoir

(Intermittent)

San

Carlos

Reservoir

East

Verde R

Pinto

Pinal CoonCr

Cr

Granite Cr

Sycamore Cre ek

Bi g

Chino Wash

Wash

Ba bocomari R

Aravaipa Creek

Beaver Cr

Wet Be aver Cr

BitterCr

French

Gulch

Jackrabbit

Wash

M ni eralCr

RillitoCr

Pantano W.

Cienega Cr

Tangle C r

Brawley Wash

El Tiro Wash

River

David son W.

Fria Agua

Quee n Creek

Oak

Creek

Dry

Beaver

Creek

Dri p

W

p

a

in

sh

g Springs W

CENTRAL

PROJECT

CANAL

ARIZONA

2.6

29

5.4

270

420

110

210

440

820

61

85

4.8

3.7

INTERMITTENT OR

EPHEMERAL STREAM

CENTRAL ARIZONA BASINS

STUDY AREA BOUNDARY

PERENNIAL STREAM

SURFACE-WATER QUALITY MONITORING

STATION

EXPLANATION

ANNUAL LOAD OF DISSOLVED

SOLIDS, IN MILLIONS OF KILOGRAMS

PER YEAR

1

1996

2 Excludes runoff

29

1, 2

Rio

Rico

Tubac

0 25 50 75 KILOMETERS

25 50 75 MILES

Figure 11. Annual stream loads of dissolved solids at selected surface-water quality monitoring stations 

in the Central Arizona Basins study area, water year 1997.
86 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

INTERMITTENT OR

EPHEMERAL STREAM

CENTRAL ARIZONA BASINS

STUDY AREA BOUNDARY

PERENNIAL STREAM

SURFACE-WATER QUALITY MONITORING

STATION

EXPLANATION

ANNUAL LOAD OF TOTAL NITROGEN, IN

MILLIONS OF KILOGRAMS PER YEAR

ANNUAL LOAD OF TOTAL PHOSPHORUS,

IN MILLIONS OF KILOGRAMS PER YEAR

1

1996

2 Excludes runoff

0

GILA

PINAL

SANTA

CRUZ

NAVAJO

GREENLEE

GRAHAM

PIMA

YAVAPAI

APACHE

GREENLEE

GRAHAM

COCHISE

COCONINO

MARICOPA

GILA

Sedona

Cottonwood

Saint

David

Nogales

Gila

Bend

PHOENIX

Eloy

Miami Globe

Payson McNary

Flagstaff

Kelvin

Green Valley

Cananea

Sierra

Vista

Camp Verde

Clarkdale

Dewey

Rock

Springs

Pinetop-Lakeside

Stanfield

Buckeye Chandler

Roosevelt

Laveen

TUCSON

Prescott

ARIZONA

MEXICO

Apache

Lake

C r Cr

Wash

Cr

Hassayampa River

New

R

Verde

River

West Clear Cr

Cave Cr

Syc amoreCr

Tonto Cr

C herry Creek

Cibeque Cr

Canyon Creek

Carri zo

Cr

River Whitee

Black

Rive

B

r

onito

Gila

River

Santa

Cruz

Altar Wash

Waterman Wash Santa Rosa

San

Pedro

River

River

Santa Cruz Wash Salt

River

Theodore

Roosevelt

Lake

Lake Pleasant

Painted

Rock

Reservoir

(Intermittent)

San

Carlos

Reservoir

East

Verde R

Pinto

Pinal CoonCr

Cr

Granite Cr

Sycamore Cre ek

Bi g

Chino Wash

Wash

Ba bocomari R

Aravaipa Creek

Beaver Cr

Wet Be aver Cr

BitterCr

French

Gulch

Jackrabbit

Wash

M ni eralCr

RillitoCr

Pantano W.

Cienega Cr

Tangle C r

Brawley Wash

El Tiro Wash

River

David son W.

Fria Agua

Quee n Creek

Oak

Creek

Dry

Beaver

Creek

Winkelman

Dri p

W

pi

a

ng Springs

sh

CENTRAL

PROJECT

CANAL

ARIZONA

1.5

.21

.36

.12

1.5

.21

.23

.058 .024

.002

.60

.018

.43

.10 1.3

.45

2

.96

2

.48

2.0

.28

.12

.027

.003

.003

1, 2

1, 2

1, 2

1, 2

Rio

Rico

Tubac

.046

.025

.002

.008

0 25 50 75 KILOMETERS

25 50 75 MILES

Figure 12. Annual stream loads of total nitrogen and total phosphorus at selected surface-water quality 

monitoring stations in the Central Arizona Basins study area, water year 1997.
Relation of Stream Loads to Upstream Conditions 87

The decrease in the stream load of phosphorus 

probably is a result of phosphorus adsorbing onto 

sediments in the streambed or in irrigated fields. If the 

phosphorus is being stored on the sediment, the 

phosphorus could accumulate in this reach during 

several consecutive dry years until high streamflow 

suspends and transports the sediment and phosphorus 

downstream. 

Seasonal patterns in stream loads of nitrogen at

stations on unregulated reaches are similar to seasonal 

patterns in streamflow (Salt River near Roosevelt, 

fig. 13); stream loads are much greater during high 

flow than during low flow. Stream loads of nitrogen 

at stations on effluent-dependent reaches also follow 

the seasonal patterns of flow (Hassayampa River, 

fig. 13); however, the seasonality is less amplified 

for effluent-dependent reaches than for unregulated 

reaches. The amplification is smaller because runoff 

tends to decrease concentrations of nitrogen in effluentdependent reaches and increase concentrations of 

nitrogen in unregulated reaches (table 7). For the Salt 

River at Roosevelt, like other stations on unregulated 

perennial streams, more of the stream load of nitrogen 

occurs as ammonia and organic nitrogen than as nitrate 

(fig. 13). For the Hassayampa River, which receives 

agricultural effluent, the opposite is true—more of 

the stream load of nitrogen occurs as nitrate than 

as ammonia and organic nitrogen (fig. 13).

An analysis of the annual mass of nitrogen and 

phosphorus from major sources that are input to

the land surface of a basin and of the mass that is 

transported out of the basin in the annual stream load 

provides information about the relative importance of 

each major source of the nutrients and the fate of the 

nutrients in that basin. The portion of the inputs that is 

not transported out of the basin in streamflow is either 

transported to the subsurface (soil, unsaturated zone, 

or aquifer), released to the atmosphere (such as volatilized ammonia), or incorporated into the biomass. An 

estimate of the annual inputs and outputs (stream 

loads) of nutrients for a basin, therefore, can be used to 

determine the percentage of the nutrients that is 

removed from the basin by surface-water processes and 

the percentage of the inputs that is either removed by 

other processes or stored in the basin. 

Annual inputs to the land surface from the major 

sources were estimated for the drainage basins above 

six stations: West Clear Creek, the Salt River near 

Roosevelt (the upper Salt River Basin), the Verde River 

below Tangle Creek (the upper Verde River Basin),

the San Pedro River at Charleston (the upper San Pedro 

River Basin), the Hassayampa River, and the Gila River 

at Buckeye Canal (the middle Gila River Basin). The 

middle Gila River Basin, for the purpose of this report, 

is defined as the area from Kelvin to the Buckeye Canal 

that drains to the Gila River. For the purpose of 

estimating inputs of nutrients, the drainage basin for 

the Hassayampa River includes the natural drainage 

basin plus an area of about 500 km2 north of the

Buckeye Canal between the Agua Fria and 

Hassayampa Rivers, because portions of this area drain 

into the Buckeye Canal and then into the 

Hassayampa River.

Each basin represents a different mix of land 

uses and land covers that are found throughout the 

study area (Cordy and others, 1998, figs. 9 and 10).

The West Clear Creek Basin is small and has little 

municipal or agricultural development. The upper 

Salt River Basin is much larger than the West Clear 

Creek Basin and has a few small towns and irrigated 

fields. The upper Verde River Basin contains several 

minimally developed basins, such as the West Clear 

Creek Basin, but also has several small towns and 

irrigated agricultural land along the Verde River. 

The upper San Pedro River Basin has small cities,

such as Sierra Vista, Arizona, and Cananea, Mexico, 

as well as irrigated agricultural land along the 

San Pedro River. The upper part of the Hassayampa 

River Basin is mostly undeveloped and comparable

to the upper Salt River and upper Verde River Basins. 

The lower part of the Hassayampa River Basin, 

however, has towns and several thousand hectares 

of intensive agriculture. The middle Gila River Basin 

is a large basin and contains the Santa Cruz, Salt, 

Verde, and Agua Fria River Basins. The middle 

Gila River Basin generally reflects conditions of the 

whole study area and encompasses several large cities, 

such as Phoenix and Tucson, and several hundred

thousand hectares of agricultural lands. 
88 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

0

20,000

40,000

60,000

80,000

0

50

100

150

200

DISCHARGE

AMMONIA PLUS ORGANIC NITROGEN

0

20,000

40,000

60,000

80,000

0

2

4

6

8

10

26

28

30

1996 1997 1998

1996 1997 1998

MONTHLY STREAM LOAD AS NITROGEN, IN KILOGRAMS PER MONTH

MEAN DAILY DISCHARGE, IN CUBIC METERS PER SECOND

HASSAYAMPA RIVER (Effluent-dependent)

SALT RIVER NEAR ROOSEVELT (Unregulated perennial)

BREAK IN SCALE

NITRATE

Figure 13. Mean daily discharge and monthly stream load of nitrate and ammonia plus organic nitrogen at the Salt River near 

Roosevelt and at the Hassayampa River, Central Arizona Basins study area, 1996–98.
Relation of Stream Loads to Upstream Conditions 89

Annual inputs to basins from the following major 

sources of nitrogen and phosphorus were estimated: 

atmospheric deposition, fertilizer, bodily waste from 

livestock on feed, bodily waste from humans, surfacewater inflow, and toxic chemical releases. Estimates 

generally were determined by using empirical methods 

and existing data from other State and Federal agencies 

(appendix 2). The data used to estimate the annual 

inputs were not available for every year (1996, 1997, 

and 1998), and in some cases the data were only 

available for years before 1996. For this reason, the 

most recent data available were used to estimate the 

annual input for one year, and this estimate was

considered to reflect typical current (1998) conditions. 

The uncertainty of the input estimates is high because 

the coefficients used in the empirical methods to estimate the nutrient inputs were not calibrated to local 

conditions. In addition, there are other possible 

sources of nitrogen and phosphorus, such as geological 

deposits, landfills, pets, and desert legumes; however, 

methods for estimating annual inputs from these 

sources without additional data collection were not 

available. Because the estimates may not include inputs 

from all sources, the annual input estimates should be 

considered as minimums, and the results of this study 

should be considered exploratory. 

Total quantified annual inputs of nitrogen 

ranged from 0.15 million kilograms per year for the 

West Clear Creek Basin to 85 million kilograms 

per year for the middle Gila River Basin (table 14). 

Total quantified annual inputs of phosphorus ranged 

from 0.004 million kilograms per year for the West 

Clear Creek Basin to 23 million kilograms per year for 

the middle Gila River Basin (table 14). 

Table 14. Estimated annual input of nitrogen and phosphorus from various sources to drainage basins of selected surface-water quality 

monitoring stations in the Central Arizona Basins study area

Source

Central Highlands Basin and Range Lowlands

Upper Salt River 

Basin

West Clear Creek 

Basin

Upper Verde 

River Basin

Upper San Pedro 

River Basin

Hassayampa 

River Basin

Middle Gila River 

Basin

Phosphorus, in millions of kilograms per year

Human waste

Sewer

Septic

.032

.029

0

.004

.10

.10

.13

.069

.026

.027

4.1

.66

Livestock, 

nongrazing .037 0 .04 .001 .13 5.5

Fertilizer .053 0 .11 .028 .68 12.5

Industry 0 0 0 0 0 .34

Surface-water inflow 0 0 0 0 .46 .17

Unquantified unknown unknown unknown unknown unknown unknown

Total quantified

Input

Flux1

.15

14

.004

6

.36

24

.23

72

1.3

330

23

320

Nitrogen, in millions of kilograms per year

Precipitation 2.3 .14 2.6 .50 .50 11

Human waste

Sewer

Septic

.089

.082

0

.011

.28

.30

.37

.19

.072

.077

11

1.9

Livestock, 

nongrazing .13 0 .23 .004 .75 20

Fertilizer .17 0 .35 .089 2.2 40

Industry 0 0 0 0 0 .14

Surface-water inflow 0 0 0 0 1.15 .51

Unquantified unknown unknown unknown unknown unknown unknown

Total quantified

Input

Flux1

2.7

250

.15

240

3.8

250

1.2

360

4.7

1,200

85

1,200

1Units are in kilograms per square kilometer per year.
90 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

The variability in annual inputs for the basins is, 

in part, related to the basin size, and so the annual 

inputs are normalized to area and called annual

input fluxes. Total quantified annual input fluxes 

of nitrogen and phosphorus were similar for the 

upper Salt River, West Clear Creek, and upper 

Verde River Basins—about 250 kg/km2 for nitrogen 

and about 15 kg/km2 for phosphorus (table 14). 

The similarity of the total quantified annual input 

fluxes of nitrogen and phosphorus for the upper 

Salt River and upper Verde River Basins to those of 

the minimally developed West Clear Creek Basin 

suggests that, at the basin scale, the small amount 

of municipal and agricultural development present 

in the upper Salt River and upper Verde River Basins 

does not greatly change the input flux. Total quantified 

annual input fluxes for the Hassayampa River and 

the middle Gila River Basins were also similar—about 

1,200 kg/km2 for nitrogen and 330 kg/km2 for 

phosphorus. Total quantified annual input fluxes of 

nitrogen and phosphorus for the upper San Pedro 

River Basin were somewhat higher than those for the 

upper Salt River, West Clear Creek, and upper Verde 

River Basins, but were considerably lower than those 

for the Hassayampa River and middle Gila River 

Basins.

In the upper Salt River, West Clear Creek, upper 

Verde River, and upper San Pedro River Basins, precipitation was the largest quantified input of nitrogen, 

and fertilizers and bodily waste from humans were 

the largest quantified inputs of phosphorus (table 14). 

In the Hassayampa River and middle Gila River 

Basins, fertilizer was by far the largest quantified input 

of nitrogen and phosphorus. In the upper San Pedro 

River and middle Gila River Basins, more nutrient 

inputs from human bodily waste were treated by sewer 

systems than by septic tanks, whereas in the upper 

Salt River, West Clear Creek, upper Verde River, and 

Hassayampa River Basins, an approximately equal or 

larger amount of nutrient input from human bodily 

waste was treated by septic tanks than by sewer 

systems. The nutrients entering the middle Gila River 

in surface-water inflow were nearly negligible compared to the total quantified inputs. In contrast, 

nutrients from the 91st Avenue Wastewater-Treatment 

Plant releases and from agricultural return flows that 

are carried into the Hassayampa River Basin as 

surface-water inflow through the Buckeye Canal were 

a considerable part of the total quantified inputs. 

Release of nutrients by industry directly to the land

surface of basins was low or nonexistent in all basins. 

Data from the toxic chemical release inventory 

(Arizona Department of Environmental Quality, 1997), 

however, indicate that the annual mass of nitrate and 

ammonia released to the atmosphere by industry is 

much greater than that released to the land surface. 

Some of these atmospheric releases could indirectly 

become basin inputs through atmospheric deposition. 

Precipitation, fertilizer, and human bodily wastes 

that are treated by septic tanks are nonpoint sources 

of nutrients, whereas releases from industry, bodily 

wastes from livestock on feed, and human bodily 

wastes that are treated by sewer systems are point 

sources. With the exception of phosphorus inputs to 

the upper San Pedro River Basin, quantified inputs 

from nonpoint sources of nitrogen and phosphorus 

were greater than quantified inputs from point sources. 

This dominance of nonpoint sources indicates that 

strategies to protect water resources from nitrogen or 

phosphorus contamination should target inputs from 

both point and nonpoint sources.

Temporal trends in inputs from major sources 

of nitrogen and phosphorus to the State of Arizona 

can be used to approximate the temporal trends in

the inputs to the middle Gila River Basin because 

much of the agricultural land and population of 

the State is in this basin. Inputs of nitrogen and 

phosphorus to Arizona were estimated using 

methods described in Appendix 2. Inputs of nitrogen 

and phosphorus to Arizona from fertilizer grew 

steadily from the time fertilizer was commercially 

introduced in the early 1940s until the 1970s and 

have been several times greater than inputs from 

bodily waste from humans, dairy cattle, or cattle on 

feed during this period (fig. 14). Inputs of nitrogen 

and phosphorus from bodily wastes from humans 

have steadily increased over time as a result of

population growth. Inputs of nitrogen and phosphorus 

from beef cattle on feed have steadily declined since 

the 1970s.
Relation of Stream Loads to Upstream Conditions 91

40

0

10

20

30

120

0

40

20

60

80

100 NITROGEN

PHOSPHORUS

MILLION KILOGRAMS PER YEAR

1860 1940 1960 1980 2000 1880 1900 1920

ESTIMATED ANNUAL INPUTS

CATTLE ON FEED

FERTILIZER DAIRY CATTLE

HUMANS

EXPLANATION

Figure 14. Temporal trends of estimated annual inputs of nitrogen and phosphorus to Arizona from fertilizer 

and from bodily waste from cattle on feed, dairy cattle, and humans.

The percentage of nitrogen input to basins that was 

transported out of the basins in streamflow in 1996–98 

was low for all basins and ranged from 1 to 21 percent 

(table 15). The percentage of phosphorus input to 

basins that was transported out of the basins in streamflow in 1996–98 typically was higher than the percentage of nitrogen and ranged from 1 to 50 percent 

(table 15). The low percentages indicate that much of 

the nitrogen and phosphorus inputs are not being transported out of the basins in surface water. Much of the 

mass input to the basin, therefore, must be transported 

to the subsurface (soil, unsaturated zone, or aquifer), 

released to the atmosphere (such as volatilized ammonia), or incorporated into the biomass of the basin.
92 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

For the 3 years studied, 1996–98, the percentage of 

nutrient inputs that was transported out of the upper 

Salt River and upper Verde River Basins was greatest in 

1998; this was mostly a result of the larger amount of 

streamflow that occurred during those years than 

during other years (table 11). This increase in the percentage suggests that nutrient inputs could accumulate 

and remain immobile in the basin for several years until 

they are transported out of the basin during infrequent 

but large flows.

During 1996–98, the percentage of nitrogen inputs 

that were transported out of the basin was on average 

the greatest for the Hassayampa River Basin; this 

could be because the Hassayampa River is an effluentdependent stream. In contrast, the percentage of phosphorus inputs that was transported out of the Hassayampa River Basin was lower than those for the 

upper Salt River or West Clear Creek Basins during 

most years. This difference could indicate that there are 

substantial inputs of phosphorus from unquantified 

sources in the upper Salt River and West Clear Creek 

Basins, or that the physical, chemical, or biological 

processes of retaining phosphorus are more efficient 

in the Hassayampa River Basin than in the upper 

Salt River or West Clear Creek Basins.

SUMMARY AND CONCLUSIONS

A comparison of stream properties and waterchemistry constituent concentrations to water-quality 

standards, the relation of stream properties and waterchemistry constituent concentrations to natural and 

human factors, and water-chemistry constituent 

stream loads for the surface-water resources of the 

study area were analyzed using water-quality data 

collected by the NAWQA and other USGS programs. 

Stream temperature, pH, dissolved-oxygen, dissolvedsolids, suspended-sediment, and nutrient data 

collected at 41 surface-water quality monitoring 

stations through water year 1998 were used in this 

analysis. Conclusions about the water quality of 

surface-water resources presented in this report were 

made in consideration of only the stream properties 

and water-chemistry constituents investigated; other 

water-chemistry constituents not considered in this 

study could be of more relevance to the water quality of 

a given stream reach. 

Comparison of data to water-quality standards 

resulted in identification of several instances where 

stream properties or water-chemistry constituent 

concentrations exceeded Maximum Contaminant 

Levels. In a few samples from the White River, the 

Black River, and the Salt River below Stewart 

Table 15. Annual stream load of nutrients for 1996–98 as a percentage of the estimated annual inputs for selected basins in the Central 

Arizona Basins study area

[---, not computed]

Basin

Annual stream load of nutrients, as a percentage of the 

estimated annual inputs to the basin

 Phosphorus Nitrogen 

1996 1997 1998 1996 1997 1998

Central Highlands

Upper Salt River 10 17 45 3 4 7

West Clear Creek 25 50 --- 1 5 ---

Upper Verde River 3 7 47 1 1 21

Basin and Range Lowlands

Upper San Pedro River 3 1 2 --- --- ---

Hassayampa River 15 8 11 18 9 11

Middle Gila River --- 2 2 --- 2 1
Summary and Conclusions 93

Mountain Dam, pH was higher than allowed by the 

State of Arizona standard for reaches designated as 

domestic drinking water sources. State of Arizona 

standards for dissolved oxygen were typically met at 

most stations, with the exception of the Santa Cruz 

River at Cortaro, where half of the samples did not 

meet the standard. More than half of the samples

from the Salt River below Stewart Mountain Dam 

and almost all of the samples from the stations on

the Central Arizona Project Canal—two of the three 

most important surface-water sources used for drinking 

water—exceeded the USEPA drinking-water SMCL 

for dissolved solids. There were no exceedences at any 

stations where State of Arizona standards for nitrate 

were applicable, nor were there any exceedences of 

State of Arizona standards for ammonia in reaches 

designated for aquatic and wildlife warm water or 

cold water fisheries. Two reach-specific standards 

for nutrients established by the State of Arizona were 

exceeded: (1) the annual mean concentration of total 

phosphorus was exceeded in several years at stations 

on the main stems of the Salt and Verde Rivers, and

(2) the annual mean concentration of total nitrogen 

was exceeded during several years at the Salt River 

near Roosevelt and the Salt River below Stewart 

Mountain Dam.

Stream properties and water-chemistry constituent 

concentrations were found to be related to streamflow, 

season, water management, stream permanence, and 

land and water use. At stations on unregulated perennial and intermittent reaches, correlations of stream 

properties and water-chemistry constituent concentrations with streamflow were common and generally 

were consistent in sign among stations for each 

constituent (positively or negatively correlated). 

Because streamflow in unregulated perennial reaches 

was seasonally variable and the stream properties and 

water-chemistry constituent concentrations were 

correlated with streamflow, the stream properties and 

water-chemistry constituent concentrations also were 

seasonally variable. For effluent-dependent reaches, the 

correlation of stream properties and water-chemistry 

constituent concentrations with streamflow was 

controlled by the source of the effluent, the distance 

from the source to the sampling station, and the season 

at the time of sampling. Correlations of stream properties and water-chemistry constituent concentrations 

with streamflow at stations on regulated reaches generally were weaker than those for stations on 

unregulated reaches, and the signs of the correlations 

were not as consistent. In cases where stream properties and water-chemistry constituent concentrations 

were correlated with streamflow, changes in the seasonality of streamflow caused by storing water in large 

reservoirs also changed the seasonality of the stream 

properties and water-chemistry constituent concentrations. Concentrations of dissolved solids in the Salt 

River below Stewart Mountain Dam had a 7-year sinusoidal pattern as a result of variability in flows entering 

the upstream reservoir. Because the concentrations of 

dissolved solids in storm runoff and snowmelt entering 

the reservoir system were lower than concentrations in 

base flow, the concentrations in water released from the 

reservoirs decreased during or slightly after a year in 

which there was a large inflow, and then slowly 

increased until another year of large inflow. For the 

reservoir system on the Salt River, only large-volume 

flows lowered the concentration of dissolved solids 

in reservoir releases below the USEPA SMCL 

(500 mg/L). Stream properties and water-chemistry 

constituent concentrations also were found to be 

related to upstream factors. The pH, dissolved-oxygen 

percent saturation, and nutrient concentrations were 

dependent on stream regulation, stream permanence, 

and upstream disposal of wastewater. 

Storage of water in the reservoirs on the Salt and 

Verde Rivers resulted in a decrease in pH and decreases 

in concentrations of dissolved solids, suspended

sediment, and total phosphorus. These decreases 

represented improvements in water quality for

drinking-water purposes. In contrast, several species 

of nitrogen increased, which represents a degradation 

of water quality for drinking-water purposes. The 

diversion and use of surface water downstream from 

the reservoirs on the Agua Fria, Gila, Salt, and Verde 

Rivers for municipal and agricultural purposes resulted 

in several kilometers of dry streambeds, and the water 

returned from these uses was of poorer quality than 

water from upstream of the diversions. Change in the 

quality of water from before use to after use reflected 

the effects of both land and water use. Decreases in

dissolved-oxygen percent saturation and increases in 

dissolved-solids and nutrient concentrations indicated 

that water quality in the Salt River degraded as a result 

of municipal water use in the Phoenix metropolitan 

area. Stream properties and water-chemistry 

constituent concentrations in effluent-dependent 

reaches were related to the source of the wastewater.
94 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Concentrations of ammonia, total ammonia plus 

organic nitrogen, and total nitrogen were higher and 

concentrations of nitrate and dissolved oxygen were 

lower in wastewater from secondary treatment plants 

than in wastewater from secondary treatment plants 

with nitrification/denitrification. The pH, dissolvedoxygen percent saturation, and concentration of nitrate 

were higher in agricultural wastewater than in treated 

municipal wastewater. Comparison of data for West 

Clear Creek, a station in a minimally developed basin, 

with data for stations on unregulated perennial reaches 

in basins that had some rangeland, agricultural, and 

(or) municipal development, indicated that these land 

uses may increase the concentrations of dissolved 

solids, suspended sediment, total phosphorus, nitrate, 

ammonia and organic nitrogen, and total nitrogen. 

Temporal trends in streamflow and in stream properties and water-chemistry constituent concentrations 

were common in the study area. Temporal trends of 

monthly 3-day low flow, monthly mean daily flow, and 

monthly maximum daily flow in unregulated perennial 

reaches draining the Central Highlands tended to be 

higher from 1900 through the 1930s, lower from the 

1940s through the 1970s, and higher again after the 

1970s. This pattern in streamflow follows that observed 

for the mean annual precipitation for the Southwestern 

United States and indicates that long-term trends in 

flow of streams draining the Central Highlands were 

driven by long-term trends in climate. For the San 

Pedro River at Charleston, which is in the Basin and 

Range Lowlands, results from this study corroborate 

the declining trend in streamflow found by Pool and 

Coes (1999), who stated that the decline could be a 

result of declining runoff and recharge near the river 

during June through October and increased interception of ground-water flow to the river by wells and 

phreatophytes. Stream-flow increased over the period 

of record at stations on effluent-dependent reaches as a 

result of the increase in the urban population and 

associated wastewater-treatment plant outflows to the 

Salt River and the Gila River in the Phoenix metropolitan area and to the Santa Cruz River in the Tucson 

metropolitan area. 

Trends in stream properties and water-chemistry 

constituent concentrations were examined at eight 

water-quality monitoring stations in the study area and 

generally indicate that water quality has improved over 

time. Stream pH increased at the Salt River below 

Stewart Mountain Dam, the Verde River below Bartlett 

Dam, the Gila River at Kelvin, and the Gila River at 

Gillespie Dam, but had no trend at the Salt River near 

Roosevelt, Wet Bottom Creek, the Verde River below 

Tangle Creek, and the Agua Fria River near Rock 

Springs. Concentrations of dissolved oxygen increased 

at the Salt River near Roosevelt, the Salt River below 

Stewart Mountain Dam, and the Verde River below 

Tangle Creek. Concentrations of dissolved oxygen 

had no trend at the other stations. Concentrations of 

dissolved solids decreased in the Salt River below 

Stewart Mountain Dam and in the Verde River below 

Bartlett Dam. This decrease represents an improvement 

in these important drinking-water sources because 

concentrations in the Salt River typically are high and 

exceed the USEPA SMCL. This decreasing trend in 

concentrations most likely resulted from a concurrent 

increasing trend in the amount of runoff entering the 

reservoirs. Concentrations of nutrients generally have 

decreased over time at several stations. The decrease in 

concentrations of total phosphorus and total nitrogen 

over the period of sampling at the Gila River at 

Gillespie Dam resulted from an increase in streamflow 

and also from a decrease in the loads of these nutrients 

to the stream from natural and human sources and (or) 

an increase in nutrient uptake by aquatic biota and 

riparian vegetation.

Stream loads of water-chemistry constituents at 

different locations along the streams were compared 

amongst each other, and stream loads were compared 

with upstream inputs of the constituent from natural 

and anthropogenic sources to provide information 

about the relative importance of the different sources 

and about the fate of the water-chemistry constituent. 

Approximately 1.2 billion kilograms of dissolved 

solids per year that were transported into the Basin 

and Range Lowlands from the Central Arizona Project 

Canal and streams draining the Central Highlands 

accumulated in the soils, unsaturated zones, and 

aquifers in agricultural and urban areas of the Basin 

and Range Lowlands from irrigation of crops and 

urban vegetation. In streams that drain the Central 

Highlands, salt inputs from precipitation accounted

for 4 to 40 percent of the annual stream loads of 

dissolved solids, and the annual stream load increased 

with annual streamflow because of increased inputs 

from precipitation. Stream loads of dissolved solids 

increased between the 91st Avenue WastewaterTreatment Plant outfall and the Gila River at Gillespie 

Dam as a result of saline ground-water inflow and

irrigation return flows. 
References Cited 95

The total annual input fluxes from quantifiable 

sources of nitrogen and phosphorus were considerably 

higher for developed basins than for minimally developed basins. These inputs exclude unquantifiable 

sources, such as geologic formations and soils. The 

total annual input fluxes for quantifiable sources of 

nitrogen and phosphorus for the upper Salt River and 

upper Verde River Basins were similar to those of the 

West Clear Creek Basin and suggested that, at the 

basin scale, the small amount of municipal or agricultural development present in the upper Salt and upper 

Verde River Basins did not greatly change the total 

input flux to the basin. For minimally developed 

basins, precipitation was the largest quantifiable source 

of nitrogen, whereas inputs from human bodily waste 

and from fertilizers was the largest quantifiable source 

of phosphorus. This was in contrast to developed 

basins, for which fertilizer was the largest source of 

both nutrients. For most basins examined, quantifiable 

inputs of nitrogen and phosphorus from nonpoint 

sources were greater than quantifiable inputs from 

point sources. This difference emphasizes the importance of land and water management policies that protect surface-water resources from nonpoint sources, as 

well as point sources, of nutrients. The amount of nitrogen and phosphorus transported out of the basins was 

much smaller than the amount of quantifiable inputs. 

This indicated that most of the nutrients input to basins 

were not transported out in surface water, but were 

transported to the subsurface (soil, unsaturated zone, or 

aquifer), released to the atmosphere (such as volatilized 

ammonia), or incorporated into the biomass. 

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Sellers, W.D., and Hill, R.H., eds., 1974, Arizona climate, 

1931–72: Tucson, University of Arizona Press, 616 p.

Sisterson, D.L. 1990, Detailed SOx-S and NOx-N mass 

budgets for the United States and Canada, in National 

Acid Precipitation Assessment Program, Report 8, 

Acidic deposition—State of science and technology: 

National Acid Precipitation Program, Appendix A,

10 p. 
References Cited 97

SPSS, Inc., 1997, SYSTAT 7.0—Statistics: Chicago, Illinois, 

SPSS Inc., 751 p.

State of Arizona, 1996, Water quality standards: Phoenix, 

State of Arizona, Arizona Administrative Code, 

Title 18, Chap. 11, p. 1-60.

Tadayon, Saeid, Duet, N.R., Fisk, G.G., McCormack, H.F., 

Partin, C.K., Pope, G.L., and Rigas, P.D., 1999, Water 

resources data, Arizona, water year 1998: U.S. 

Geological Survey Water-Data Report AZ–98–1, 454 p.

Tadayon, Saeid, Duet, N.R., Fisk, G.G., McCormack, 

H.F., Pope, G.L., and Rigas, P.D., 1998, Water 

resources data, Arizona, water year 1997: U.S. 

Geological Survey Water-Data Report AZ–97–1, 416 p.

Trentberth, K.E., 1991, Climate change and climate 

variability: The climate record, in Managing water 

resources in the West under conditions of climate 

uncertainty: National Resources Council, Proceedings 

of the Colloquium, November 14–16, 1990, Scottsdale, 

Arizona, National Academy Press, Washington, D.C.

U.S. Department of Agriculture, 1992, National engineering 

handbook—Agricultural waste management field 

handbook, part 651: U.S. Department of Agriculture, 

Soil Conservation Service, v.p.

U.S. Department of Commerce, 1992, 1990 Census of 

population and housing—Summary social, economic, 

and housing characteristics– Arizona: U.S. Department 

of Commerce report 1990 CPH–5–4, v.p. 

———1994, 1992 Census of agriculture, Volume 1 

Geographic Area Series, Part 3 Arizona State and 

County Data: U.S Department of Commerce Report 

AC92–A–3, v.p. 

U.S. Environmental Protection Agency, 1980, Design 

manual: On site wastewater treatment and disposal 

systems: U.S. Environmental Protection Agency, Office 

of Water Program Operations, Research and 

Development, Municipal Environmental Research 

Laboratory, 391 p. 

____1996, Drinking water regulations and health advisories: 

U.S. Environmental Protection Agency, Office of 

Drinking Water, 11 p.

U.S. Federal Water Pollution Control Administration, 1968, 

Report of the committee on water-quality criteria: 

Washington, D.C., 234 p.

U.S. Public Health Service, 1962, Drinking water standards, 

1962: U.S. Health Service Publication 956, 61 p. 

Wilcoxon, F., 1945, Individual comparisons by ranking 

methods: Biometrics, v. 1, p. 80–83.
98 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998
Appendix 1 99

APPENDIX 1:

STATISTICAL SUMMARIES OF STREAM PROPERTIES AND WATER-CHEMISTRY 

CONSTITUENTS AT SURFACE-WATER QUALITY MONITORING STATIONS IN 

THE CENTRAL ARIZONA BASINS STUDY AREA, THROUGH 1998

Appendix 1 101

Appendix 1. Statistical summaries of stream properties and water-chemistry constituents at surface-water quality monitoring stations in the 

Central Arizona Basins study area, through 1998

[mg/L, milligrams per liter; ---, not computed; <, value is less than the number listed; WWTP, wastewater treatment plant]

Station Minimum

Percentile

Maximum Mean

Standard 

deviation

Number of 

25th Median 75th samples

Stream temperature, in degrees Celsius

Central Highlands

Black River 2 9 17 23 29 16 8 44

White River 2 5.5 13.5 20.5 27 14 8 45

Pinal Creek 10 17 22 26 34 22 6 160

Salt River near Roosevelt 4.5 10.5 17 24 30 17 7.5 265

Tonto Creek 2 11 19.5 26 31 18.5 8 55

Salt River below Stewart Mountain Dam 8.5 14 18 20.5 25.5 17.5 4 173

Verde River near Clarkdale 5 12 18 22 28.5 17 6 192

Verde River near Cornville 6 10 15 23 27 16 7 24

Oak Creek near Sedona 1 7 11 15 20 11 5 23

Oak Creek at Red Rock Crossing 3.5 8 14 21 28 14.5 7 147

Oak Creek near Cornville 6 12.5 20 23 29 18.5 7 34

Verde River above West Clear Creek 6.2 13.5 16.5 25 26.5 18 6.5 21

West Clear Creek 5.5 9 14 22 27 15 7 46

Verde River near Camp Verde 5 10 17 24 35 17 7.5 112

East Verde River 6 10 17 24 30 17.5 8 47

Wet Bottom Creek 5 11 16 22.5 32 16.5 7 181

Verde River below Tangle Creek 5 12.5 19 25.5 31.5 19 7 199

Verde River below Bartlett Dam 7 12 15 19 29 16 5 191

Turkey Creek 7 9 20.5 22.5 29 17.5 7.5 40

Agua Fria River near Rock Springs 3.5 13 20 25 33 19 6.5 167

Agua Fria River below Waddell Dam 10 14.5 16.5 19 29 17 3.5 75

Basin and Range Lowlands

Central Arizona Project near Parker 11 15 20 25.5 30 20 6 41

Central Arizona Project at Phoenix 8.5 14 20 26 31 20 6.5 123

Gila River at Winkelman 6 12.5 16 19 32 16.5 5.5 86

San Pedro River at Charleston 2 16 21 24.5 32 20.5 6 176

San Pedro River below Aravaipa 7 18 22 26.5 33.5 22 5.5 115

San Pedro River at Winkelman 6 19 25 30.5 39 24.5 8 114

Gila River at Kelvin 7 13.5 19 24 32 19 6.5 305

Santa Cruz River at Rio Rico 13 21 23.5 26 29 23 4 26

Santa Cruz River at Tubac 12 17 22 24 26.5 21 4 27

Santa Cruz River at Cortaro 17.5 24 26 28 31 26 4 14

Santa Cruz River at Laveen 6 11 14.5 21 25 15.5 6.5 16

Indian Bend Wash at Curry Road 10 14 17 27 32 20 7 19

Box Culvert at 48th Street Drain 11.5 15.5 19 27.5 31.5 21 7 25

27th Avenue at Salt River 12.5 15 17 26.5 29 19.5 5.5 28

43rd Avenue and Peoria Avenue 11 14.5 18.5 27.5 30 20.5 7 25

Olive Avenue and 67th Avenue 11 13 17 27 28 19 7 18

91st Avenue WWTP outfall 23 25 28 30 33 27.5 3.5 24

Gila River at Buckeye Canal 15 19 21.5 26.5 31 22.5 4.5 40

Hassayampa River 12 17.5 23 26.5 32.5 22 5 48

Gila River at Gillespie Dam 8 14 20.5 26 34 20 6.5 280
102 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Appendix 1. Statistical summaries of stream properties and water-chemistry constituents at surface-water quality monitoring stations in the Central 

Arizona Basins study area, through 1998—Continued

Station Minimum

Percentile

Maximum Mean

Standard 

deviation

Number of 

25th Median 75th samples

Stream pH, in standard units

Central Highlands

Black River 6.9 7.4 7.9 8.3 9.6 8.0 0.6 41

White River 6.4 7.9 8.3 8.5 11.0 8.2 .7 44

Pinal Creek 5.7 7.7 7.9 8.0 8.4 7.8 .4 158

Salt River near Roosevelt 6.9 8.1 8.3 8.4 9.2 8.2 .3 264

Tonto Creek 7.2 8.0 8.2 8.4 8.9 8.2 .4 55

Salt River below Stewart Mountain Dam 4.5 7.6 7.8 8.0 9.1 7.8 .4 496

Verde River near Clarkdale 6.9 8.1 8.2 8.3 8.6 8.2 .2 188

Verde River near Cornville 7.7 8.1 8.2 8.3 8.6 8.2 .2 24

Oak Creek near Sedona 7.7 8.2 8.4 8.4 8.7 8.3 .2 20

Oak Creek at Red Rock Crossing 6.9 8.2 8.3 8.5 8.8 8.3 .3 141

Oak Creek near Cornville 7.1 8.0 8.1 8.3 8.7 8.1 .3 35

Verde River above West Clear Creek 7.9 8.3 8.3 8.4 8.7 8.3 .2 21

West Clear Creek 6.8 8.1 8.4 8.5 8.7 8.2 .5 47

Verde River near Camp Verde 6.4 8.2 8.3 8.4 9.2 8.3 .3 108

East Verde River 7.8 8.3 8.4 8.5 8.6 8.4 .2 46

Wet Bottom Creek 6.5 7.6 7.9 8.1 9.4 7.8 .4 169

Verde River below Tangle Creek 6.5 8.3 8.4 8.5 9.0 8.4 .3 198

Verde River below Bartlett Dam 6.8 7.9 8.1 8.2 8.9 8.1 .3 603

Turkey Creek 6.6 7.4 7.9 8.1 8.8 7.8 .5 37

Agua Fria River near Rock Springs 6.9 8.2 8.3 8.4 8.8 8.3 .3 166

Agua Fria River below Waddell Dam 7.1 8.1 8.2 8.4 8.9 8.2 .3 75

Basin and Range Lowlands

Central Arizona Project near Parker 7.8 8.0 8.2 8.2 8.4 8.2 0.1 41

Central Arizona Project at Phoenix 7.8 8.3 8.4 8.4 8.9 8.4 .2 123

Gila River at Winkelman 7.3 8.0 8.2 8.4 8.8 8.2 .3 85

San Pedro River at Charleston 7.7 8.2 8.3 8.4 8.9 8.3 .2 87

San Pedro River below Aravaipa 7.8 8.1 8.3 8.5 8.9 8.3 .3 38

San Pedro River at Winkelman 7.6 8.0 8.1 8.2 8.6 8.1 .2 38

Gila River at Kelvin 3.2 7.6 7.8 8.0 8.6 7.8 .4 996

Santa Cruz River at Rio Rico 7.6 8.0 8.2 8.3 8.6 8.1 .2 26

Santa Cruz River at Tubac 7.3 7.8 7.8 7.9 8.1 7.8 .2 28

Santa Cruz River at Cortaro 7.4 7.6 7.7 7.8 8.1 7.7 .2 13

Santa Cruz River at Laveen 7.6 7.9 8.4 9.1 9.7 8.5 .7 15

Indian Bend Wash at Curry Road 6.7 7.4 7.8 8.0 8.5 7.8 .4 19

Box Culvert at 48th Street Drain 5.9 6.9 7.1 7.6 8.3 7.2 .6 26

27th Avenue at Salt River 6.9 7.6 8.0 8.4 9.3 8.0 .6 28

43rd Avenue and Peoria Avenue 5.7 6.4 6.7 6.9 8.1 6.7 .5 25

Olive Avenue and 67th Avenue 6.5 7.0 7.3 7.7 8.3 7.3 .5 19

91st Avenue WWTP outfall 7.0 7.2 7.3 7.4 7.6 7.3 .2 24

Gila River at Buckeye Canal 7.4 7.6 7.8 7.8 8.3 7.8 .2 40

Hassayampa River 7.6 8.0 8.1 8.2 9.0 8.1 .2 49

Gila River at Gillespie Dam 6.5 7.7 7.8 8.1 9.5 7.9 .3 646
Appendix 1 103

Appendix 1. Statistical summaries of stream properties and water-chemistry constituents at surface-water quality monitoring stations in the 

Central Arizona Basins study area, through 1998—Continued

Station Minimum

Percentile

Maximum Mean

Standard 

deviation

Number of 

25th Median 75th samples

Dissolved oxygen-concentration, in mg/L

Central Highlands

Black River 6.7 7.6 8.4 9.2 14.0 8.7 1.6 35

White River 6.9 7.6 8.7 9.6 12.2 9.0 1.6 37

Pinal Creek 5.8 7.1 7.8 8.5 14.1 7.9 1.2 154

Salt River near Roosevelt 5.8 8.0 9.1 10.4 15.9 9.2 1.7 240

Tonto Creek 5.8 8.0 9.2 10.4 17.2 9.5 2.3 40

Salt River below Stewart Mountain Dam 1.6 7.2 8.5 10.0 13.7 8.5 2.3 119

Verde River near Clarkdale 4.6 8.5 9.4 10.2 12.2 9.4 1.3 176

Verde River near Cornville 6.8 8.6 9.6 10.4 11.0 9.3 1.2 20

Oak Creek near Sedona 7.4 8.9 9.6 10.8 11.7 9.8 1.2 19

Oak Creek at Red Rock Crossing 7.2 8.1 9.4 10.5 12.8 9.4 1.4 138

Oak Creek near Cornville 6.4 7.9 8.9 9.7 11.0 8.9 1.4 24

Verde River above West Clear Creek 6.7 9.0 9.8 10.5 11.8 9.7 1.3 20

West Clear Creek 7.4 8.1 9.4 10.4 11.3 9.3 1.2 38

Verde River near Camp Verde 3.3 8.4 9.4 10.7 13.0 9.5 1.6 98

East Verde River 5.8 7.6 8.6 10.3 11.6 8.9 1.6 46

Wet Bottom Creek 5.4 7.6 8.4 10.0 12.2 8.7 1.5 117

Verde River below Tangle Creek 6.2 7.9 8.8 10.2 13.6 9.0 1.4 191

Verde River below Bartlett Dam 6.6 9.2 10.3 11.4 17.8 10.4 1.9 115

Turkey Creek 8.6 10.3 10.4 10.6 11.3 10.2 1.0 5

Agua Fria River near Rock Springs 3.6 8.1 8.9 10.0 16.2 9.0 1.9 163

Agua Fria River below Waddell Dam 6.7 8.8 9.9 11.0 13.6 9.9 1.5 73

Basin and Range Lowlands

Central Arizona Project near Parker 7.6 8.6 9.2 9.9 16.2 9.4 1.5 41

Central Arizona Project at Phoenix 2.1 8.2 9.2 10.3 13.2 9.2 1.6 122

Gila River at Winkelman 4.1 8.4 9.4 10.6 13.2 9.4 1.6 77

San Pedro River at Charleston 5.6 7.3 8.6 9.6 13.5 8.6 1.7 85

San Pedro River below Aravaipa 4.2 7.2 8.2 8.8 11.6 8.1 1.5 37

San Pedro River at Winkelman 5.7 7.2 8.8 9.8 10.4 8.5 1.5 22

Gila River at Kelvin 5.4 8.0 9.0 10.1 11.3 9.0 1.4 68

Santa Cruz River at Rio Rico 5.0 6.7 7.2 7.6 10.3 7.3 1.3 20

Santa Cruz River at Tubac 3.6 4.7 5.3 5.9 9.6 5.4 1.2 27

Santa Cruz River at Cortaro 2.0 2.4 2.6 3.2 3.7 2.8 .6 12

Santa Cruz River at Laveen 4.3 7.0 9.1 10.0 12.1 8.6 2.2 14

Indian Bend Wash at Curry Road 4.8 7.8 8.6 9.1 11.2 8.2 1.9 12

Box Culvert at 48th Street Drain 3.8 6.6 7.3 8.3 11.6 7.5 1.7 19

27th Avenue at Salt River 5.2 5.8 7.6 8.7 10.2 7.5 1.6 19

43rd Avenue and Peoria Avenue 5.6 6.7 7.2 8.6 10.0 7.6 1.4 17

Olive Avenue and 67th Avenue 6.1 7.0 7.6 8.8 9.8 7.8 1.2 13

91st Avenue WWTP outfall 3.7 4.5 5.0 5.5 6.0 5.0 .6 24

Gila River at Buckeye Canal 2.9 4.7 5.6 6.5 10.9 5.8 1.7 40

Hassayampa River 6.1 8.1 9.1 9.8 13.6 9.0 1.4 43

Gila River at Gillespie Dam 3.5 6.7 8.4 9.7 17.1 8.5 2.6 222
104 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Dissolved-oxygen percent saturation, in percent

Central Highlands

Black River --- --- --- --- --- --- --- 0

White River --- --- --- --- --- --- --- 0

Pinal Creek 84 95 98 102 157 100 8 120

Salt River near Roosevelt 83 98 101 105 139 102 8 181

Tonto Creek 81 98 100 107 132 102 13 12

Salt River below Stewart Mountain Dam 69 92 100 110 153 102 16 71

Verde River near Clarkdale 91 103 110 115 140 110 9 123

Verde River near Cornville --- --- --- --- --- --- --- 0

Oak Creek near Sedona --- --- --- --- --- --- --- 0

Oak Creek at Red Rock Crossing 95 100 102 105 124 103 5 98

Oak Creek near Cornville --- --- --- --- --- --- --- 0

Verde River above West Clear Creek 89 104 112 122 149 115 16 20

West Clear Creek 97 101 102 107 113 104 5 38

Verde River near Camp Verde 88 102 106 111 120 106 6 58

East Verde River 85 96 100 102 117 100 6 46

Wet Bottom Creek 75 98 100 104 126 101 9 52

Verde River below Tangle Creek 86 100 103 107 159 104 7 170

Verde River below Bartlett Dam 79 105 109 118 144 111 11 69

Turkey Creek 101 --- --- --- 144 122 30 2

Agua Fria River near Rock Springs 47 93 100 108 235 103 22 155

Agua Fria River below Waddell Dam 81 98 105 115 155 108 15 66

Basin and Range Lowlands

Central Arizona Project near Parker 89 97 103 110 210 107 19 41

Central Arizona Project at Phoenix 21 101 107 113 142 106 15 121

Gila River at Winkelman 85 95 99 104 129 100 10 23

San Pedro River at Charleston 75 94 106 116 162 107 16 83

San Pedro River below Aravaipa 87 94 96 100 108 97 6 17

San Pedro River at Winkelman --- --- --- --- --- --- --- 0

Gila River at Kelvin 90 96 99 103 120 100 6 27

Santa Cruz River at Rio Rico --- --- --- --- --- --- --- 0

Santa Cruz River at Tubac 48 58 65 76 114 69 15 27

Santa Cruz River at Cortaro 27 31 34 40 52 37 8 12

Santa Cruz River at Laveen --- --- --- --- --- --- --- 0

Indian Bend Wash at Curry Road 63 80 94 102 115 91 17 12

Box Culvert at 48th Street Drain 44 82 88 94 123 88 15 19

27th Avenue at Salt River 54 76 89 93 104 85 13 19

43rd Avenue and Peoria Avenue 61 86 90 95 106 89 12 17

Olive Avenue and 67th Avenue 70 84 91 98 107 90 10 13

91st Avenue WWTP outfall 50 62 66 70 79 66 7 23

Gila River at Buckeye Canal 34 58 66 76 147 71 24 39

Hassayampa River 79 95 101 114 199 108 22 42

Gila River at Gillespie Dam 47 73 92 103 209 93 28 158

Appendix 1. Statistical summaries of stream properties and water-chemistry constituents at surface-water quality monitoring stations in the 

Central Arizona Basins study area, through 1998—Continued

Station Minimum

Percentile

Maximum Mean

Standard 

deviation

Number of 

25th Median 75th samples
Appendix 1 105

Dissolved-solids concentration, in mg/L

Central Highlands

Black River 51 78 92 99 124 88 16 44

White River 66 156 205 244 293 195 63 44

Pinal Creek 247 2,510 2,740 2,920 3,370 2,700 445 152

Salt River near Roosevelt 105 542 1,100 1,670 3,110 1,150 663 220

Tonto Creek 91 188 245 285 354 239 63 54

Salt River below Stewart Mountain Dam 275 435 540 696 1,270 571 176 389

Verde River near Clarkdale 59 268 280 288 310 258 58 167

Verde River near Cornville 158 348 356 364 404 340 59 23

Oak Creek near Sedona 56 113 166 172 192 143 43 23

Oak Creek at Red Rock Crossing 53 150 163 168 202 149 35 116

Oak Creek near Cornville 71 201 210 222 301 192 60 34

Verde River above West Clear Creek 134 380 420 507 667 425 124 21

West Clear Creek 51 95 190 202 215 160 59 46

Verde River near Camp Verde 73 351 429 499 651 398 156 81

East Verde River 90 187 215 242 377 221 56 47

Wet Bottom Creek 40 97 182 232 375 167 76 172

Verde River below Tangle Creek 86 332 365 392 481 340 87 159

Verde River below Bartlett Dam 102 211 305 372 604 294 88 429

Turkey Creek 91 118 177 374 1,320 300 276 25

Agua Fria River near Rock Springs 111 357 399 426 525 385 76 162

Agua Fria River below Waddell Dam 209 250 298 339 595 328 107 75

Basin and Range Lowlands

Central Arizona Project near Parker 356 529 593 660 693 592 77 41

Central Arizona Project at Phoenix 206 528 577 645 700 575 90 123

Gila River at Winkelman 242 497 773 953 2,870 822 468 50

San Pedro River at Charleston 117 270 296 316 379 286 48 83

San Pedro River below Aravaipa 310 743 835 883 1,010 760 202 37

San Pedro River at Winkelman 232 606 925 1,060 1,240 820 310 40

Gila River at Kelvin 167 572 784 1,140 2,980 926 472 406

Santa Cruz River at Rio Rico 263 387 460 496 555 443 73 26

Santa Cruz River at Tubac 116 414 423 448 460 412 71 27

Santa Cruz River at Cortaro 314 573 585 592 610 560 78 13

Santa Cruz River at Laveen 167 258 346 459 1,060 404 223 15

Indian Bend Wash at Curry Road 75 175 283 466 610 312 180 19

Box Culvert at 48th Street Drain 35 51 92 110 257 94 53 23

27th Avenue at Salt River 92 124 154 175 444 174 80 26

43rd Avenue and Peoria Avenue 18 25 40 58 183 53 41 24

Olive Avenue and 67th Avenue 26 34 47 56 90 48 17 19

91st Avenue WWTP outfall 765 866 888 912 971 886 49 24

Gila River at Buckeye Canal 1,010 1,230 1,350 1,380 1,640 1,330 128 40

Hassayampa River 98 1,220 1,740 2,210 2,630 1,670 622 48

Gila River at Gillespie Dam 144 2,080 2,740 3,810 7,230 2,940 1,440 384

Appendix 1. Statistical summaries of stream properties and water-chemistry constituents at surface-water quality monitoring stations in the 

Central Arizona Basins study area, through 1998—Continued

Station Minimum

Percentile

Maximum Mean

Standard 

deviation

Number of 

25th Median 75th samples
106 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Suspended-sediment concentration, in mg/L

Central Highlands

Black River 1 3 6 7 57 15 21 9

White River 3 7 11 30 217 36 63 11

Pinal Creek .2 7 35 169 25,560 334 2,220 133

Salt River near Roosevelt 1 11 30 151 11,690 307 1,110 208

Tonto Creek 1 5 9 17 2,190 106 445 24

Salt River below Stewart Mountain Dam 1 3 5 8 259 12 33 85

Verde River near Clarkdale 3 16 27 49 2,760 84 274 136

Verde River near Cornville --- --- --- --- --- --- --- 0

Oak Creek near Sedona 1 4 7 24 377 35 84 20

Oak Creek at Red Rock Crossing 1 4 7 12 2,400 34 217 127

Oak Creek near Cornville --- --- --- --- --- --- --- 0

Verde River above West Clear Creek 11 30 50 86 374 72 77 21

West Clear Creek 2 3 6 18 2,720 97 454 36

Verde River near Camp Verde 6 22 36 74 2,700 126 367 70

East Verde River .1 1 4 10 177 12 29 44

Wet Bottom Creek .3 1 3 6 110 7 15 118

Verde River below Tangle Creek .3 7 15 48 3,320 77 273 176

Verde River below Bartlett Dam 1 8 12 23 8,560 98 747 136

Turkey Creek 3 165 836 1,750 24,300 2,480 4,800 42

Agua Fria River near Rock Springs .2 3 6 20 29,400 439 2,580 163

Agua Fria River below Waddell Dam 2 6 8 16 151 15 23 75

Basin and Range Lowlands

Central Arizona Project near Parker --- --- --- --- --- --- --- 0

Central Arizona Project at Phoenix .2 2 4 8 478 13 50 116

Gila River at Winkelman 5 60 120 183 12,100 341 1,600 56

San Pedro River at Charleston 2 29 322 19,020 142,000 11,200 19,840 152

San Pedro River below Aravaipa 2 43 108 460 71,700 3,130 10,860 132

San Pedro River at Winkelman 47 6,340 45,700 90,500 208,000 54,840 50,030 102

Gila River at Kelvin 5 150 487 7,740 200,000 11,570 25,240 313

Santa Cruz River at Rio Rico 76 93 319 367 2,100 591 854 5

Santa Cruz River at Tubac 1 18 30 61 6,410 370 1,330 23

Santa Cruz River at Cortaro 10 18 26 66 33,600 2,440 8,970 14

Santa Cruz River at Laveen 44 2,660 5,060 6,720 15,100 5,280 3,830 14

Indian Bend Wash at Curry Road --- --- --- --- --- --- --- 0

Box Culvert at 48th Street Drain --- --- --- --- --- --- --- 0

27th Avenue at Salt River --- --- --- --- --- --- --- 0

43rd Avenue and Peoria Avenue --- --- --- --- --- --- --- 0

Olive Avenue and 67th Avenue --- --- --- --- --- --- --- 0

91st Avenue WWTP outfall 2 3 5 5 37 7 9 20

Gila River at Buckeye Canal 2 18 33 65 111 44 31 31

Hassayampa River 14 37 76 342 36,500 1,720 6,060 38

Gila River at Gillespie Dam 3 52 93 176 15,010 278 1,060 250

Appendix 1. Statistical summaries of stream properties and water-chemistry constituents at surface-water quality monitoring stations in the 

Central Arizona Basins study area, through 1998—Continued

Station Minimum

Percentile

Maximum Mean

Standard 

deviation

Number of 

25th Median 75th samples
Appendix 1 107

Dissolved-orthophosphorus concentration, in mg/L as phosphorus

Central Highlands

Black River <0.01 0.02 0.03 0.05 13 --- --- 37

White River <.01 <.01 .02 .04 17 --- --- 37

Pinal Creek <.01 .02 .03 .04 .2 --- --- 63

Salt River near Roosevelt <.01 <.01 .02 .03 .11 --- --- 143

Tonto Creek <.01 <.01 <.01 .02 .07 --- --- 49

Salt River below Stewart Mountain Dam <.01 <.01 .02 .03 .06 --- --- 72

Verde River near Clarkdale <.01 <.01 <.01 .03 .09 --- --- 51

Verde River near Cornville <.01 <.01 .02 .03 .07 --- --- 23

Oak Creek near Sedona <.01 <.01 <.01 .02 .07 --- --- 18

Oak Creek at Red Rock Crossing <.01 <.01 .02 .02 .05 --- --- 29

Oak Creek near Cornville <.01 <.01 <.01 .02 .05 --- --- 31

Verde River above West Clear Creek <.01 <.01 <.01 <.01 .04 --- --- 14

West Clear Creek <.01 <.01 <.01 .03 .79 --- --- 44

Verde River near Camp Verde <.01 <.01 .02 .03 .06 --- --- 38

East Verde River --- --- --- --- --- --- --- 0

Wet Bottom Creek <.01 <.01 <.01 .02 .07 --- --- 107

Verde River below Tangle Creek <.01 <.01 <.01 .02 .05 --- --- 86

Verde River below Bartlett Dam <.01 .02 .02 .04 .09 --- --- 87

Turkey Creek --- --- --- --- --- --- --- 0

Agua Fria River near Rock Springs <.01 .03 .04 .05 .18 --- --- 165

Agua Fria River below Waddell Dam <.01 .02 .03 .04 .09 --- --- 75

Basin and Range Lowlands

Central Arizona Project near Parker <0.01 <0.01 <0.01 <0.01 0.09 --- --- 39

Central Arizona Project at Phoenix <.01 <.01 <.01 <.01 .05 --- --- 123

Gila River at Winkelman <.01 .03 .07 .09 .15 --- --- 40

San Pedro River at Charleston <.01 <.01 <.01 .02 .10 --- --- 74

San Pedro River below Aravaipa <.01 .02 .03 .05 .13 --- --- 31

San Pedro River at Winkelman <.01 .02 .03 .05 .20 --- --- 30

Gila River at Kelvin <.01 .03 .05 .08 .25 --- --- 182

Santa Cruz River at Rio Rico .13 1.8 3.7 4.6 7.5 3.5 2.1 21

Santa Cruz River at Tubac .12 .42 .89 1.4 3.0 1.0 .72 27

Santa Cruz River at Cortaro .15 3.3 3.7 4.0 4.1 3.2 1.3 12

Santa Cruz River at Laveen --- --- .06 --- --- --- --- 1

Indian Bend Wash at Curry Road --- --- --- --- --- --- --- 0

Box Culvert at 48th Street Drain --- --- --- --- --- --- --- 0

27th Avenue at Salt River --- --- --- --- --- --- --- 0

43rd Avenue and Peoria Avenue --- --- --- --- --- --- --- 0

Olive Avenue and 67th Avenue --- --- --- --- --- --- --- 0

91st Avenue WWTP outfall 1.0 2.8 3.6 4.6 5.1 3.5 1.2 24

Gila River at Buckeye Canal 1.0 2.0 2.4 2.8 3.5 2.4 .61 40

Hassayampa River <.01 .52 1.1 2.0 3.2 1.3 .88 48

Gila River at Gillespie Dam <.01 .35 .70 1.8 3.4 1.1 .92 168

Appendix 1. Statistical summaries of stream properties and water-chemistry constituents at surface-water quality monitoring stations in the 

Central Arizona Basins study area, through 1998—Continued

Station Minimum

Percentile

Maximum Mean

Standard 

deviation

Number of 

25th Median 75th samples
108 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Total phosphorus concentration, in mg/L as phosphorus

Central Highlands

Black River <0.01 0.04 0.05 0.09 0.27 --- --- 43

White River .02 .04 .06 .09 .48 --- --- 43

Pinal Creek <.01 .03 .05 .11 10 --- --- 143

Salt River near Roosevelt <.01 .02 .04 .11 4.0 --- --- 260

Tonto Creek <.01 <.01 .02 .04 .19 --- --- 54

Salt River below Stewart Mountain Dam <.01 .02 .03 .04 8.3 --- --- 145

Verde River near Clarkdale <.01 .02 .03 .07 3.4 --- --- 188

Verde River near Cornville <.01 .05 .07 .14 .76 --- --- 23

Oak Creek near Sedona <.01 <.01 .02 .06 .23 --- --- 22

Oak Creek at Red Rock Crossing <.01 <.01 .02 .04 .70 --- --- 141

Oak Creek near Cornville <.01 .02 .04 .07 .31 --- --- 30

Verde River above West Clear Creek <.01 .02 .02 .08 .32 --- --- 20

West Clear Creek <.01 <.01 <.01 .04 .60 --- --- 39

Verde River near Camp Verde <.01 .02 .04 .08 3.0 --- --- 104

East Verde River <.01 .02 .02 .03 .16 --- --- 47

Wet Bottom Creek <.01 <.01 <.01 .02 .50 --- --- 100

Verde River below Tangle Creek <.01 .02 .03 .05 .97 --- --- 197

Verde River below Bartlett Dam <.01 .03 .05 .07 7.3 --- --- 147

Turkey Creek <.01 .14 .55 2.4 11 --- --- 43

Agua Fria River near Rock Springs <.01 .04 .05 .07 39 --- --- 166

Agua Fria River below Waddell Dam <.01 .04 .06 .09 .19 --- --- 75

Basin and Range Lowlands

Central Arizona Project near Parker <0.01 <0.01 <0.01 0.02 0.15 --- --- 41

Central Arizona Project at Phoenix <.01 <.01 <.01 .02 .25 --- --- 123

Gila River at Winkelman <.01 .09 .16 .22 7.3 --- --- 86

San Pedro River at Charleston <.01 .02 .04 .06 1.3 --- --- 86

San Pedro River below Aravaipa .02 .04 .08 .29 40 --- --- 37

San Pedro River at Winkelman .02 .05 .10 1.4 14 --- --- 40

Gila River at Kelvin <.01 .08 .15 .32 13 --- --- 92

Santa Cruz River at Rio Rico 1.1 3.4 6.1 6.9 11 5.5 2.6 26

Santa Cruz River at Tubac .15 .75 1.4 2.2 6.4 1.8 1.5 26

Santa Cruz River at Cortaro 3.4 3.9 4.1 4.4 5.2 4.2 .5 12

Santa Cruz River at Laveen .16 4.2 6.0 6.8 13 5.7 3.3 15

Indian Bend Wash at Curry Road .05 .09 .16 .33 .52 .21 .15 19

Box Culvert at 48th Street Drain .15 .33 .94 1.5 2.5 1.0 .73 23

27th Avenue at Salt River .51 .81 1.2 1.7 7.9 1.6 1.5 26

43rd Avenue and Peoria Avenue .16 .34 .49 .75 2.0 .58 .38 25

Olive Avenue and 67th Avenue .14 .38 .49 .54 1.2 .53 .28 19

91st Avenue WWTP outfall 2.2 3.4 4.3 5.0 6.1 4.2 1.0 23

Gila River at Buckeye Canal 1.6 2.5 2.9 3.2 4.0 2.9 .54 39

Hassayampa River .59 1.1 2.0 3.0 53 3.2 7.4 48

Gila River at Gillespie Dam .10 .93 1.6 2.6 9.4 1.8 1.2 250

Appendix 1. Statistical summaries of stream properties and water-chemistry constituents at surface-water quality monitoring stations in the 

Central Arizona Basins study area, through 1998—Continued

Station Minimum

Percentile

Maximum Mean

Standard 

deviation

Number of 

25th Median 75th samples
Appendix 1 109

Nitrate concentration, in mg/L as nitrogen

Central Highlands

Black River <0.01 <0.01 0.02 0.05 0.42 --- --- 44

White River <.01 <.01 .02 .08 .49 --- --- 44

Pinal Creek <.02 .09 .10 .13 1.3 --- --- 150

Salt River near Roosevelt <.01 .05 .10 .10 1.3 --- --- 254

Tonto Creek <.01 <.01 .09 .10 .29 --- --- 54

Salt River below Stewart Mountain Dam <.01 .06 .10 .10 .59 --- --- 163

Verde River near Clarkdale <.01 .07 .10 .20 .60 --- --- 179

Verde River near Cornville <.01 .07 .13 .25 .72 --- --- 23

Oak Creek near Sedona <.01 .03 .05 .07 .08 --- --- 21

Oak Creek at Red Rock Crossing <.01 .05 .10 .10 .18 --- --- 127

Oak Creek near Cornville <.01 .04 .06 .09 .34 --- --- 32

Verde River above West Clear Creek <.02 .06 .09 .11 .23 --- --- 20

West Clear Creek <.01 <.05 <.05 <.05 .60 --- --- 44

Verde River near Camp Verde <.01 <.05 .10 .10 1.3 --- --- 95

East Verde River <.02 <.02 .02 .05 .15 --- --- 42

Wet Bottom Creek <.01 .03 .05 .10 .70 --- --- 142

Verde River below Tangle Creek <.01 <.05 .10 .10 .42 --- --- 188

Verde River below Bartlett Dam <.01 .10 .10 .17 .81 --- --- 185

Turkey Creek .10 .21 .27 .55 1.2 .40 .30 25

Agua Fria River near Rock Springs <.05 .10 .10 .10 .80 --- --- 166

Agua Fria River below Waddell Dam <.05 .10 .10 .11 .46 --- --- 75

Basin and Range Lowlands

Central Arizona Project near Parker <0.05 0.10 0.17 0.20 0.36 --- --- 40

Central Arizona Project at Phoenix <.05 .07 .10 .10 .39 --- --- 123

Gila River at Winkelman <.01 .14 .26 .35 15 --- --- 82

San Pedro River at Charleston <.05 <.05 .09 .15 .83 --- --- 82

San Pedro River below Aravaipa .30 .53 .65 .80 1.8 .70 .27 37

San Pedro River at Winkelman .18 .48 .75 .98 4.0 .85 .64 37

Gila River at Kelvin <.01 .12 .29 .62 2.7 --- --- 200

Santa Cruz River at Rio Rico <.02 .36 .59 2.7 3.9 --- --- 25

Santa Cruz River at Tubac .25 1.6 2.3 2.8 4.7 2.2 1.1 27

Santa Cruz River at Cortaro .06 .13 .33 .46 1.5 .43 .44 12

Santa Cruz River at Laveen .28 1.0 1.2 1.7 4.1 1.6 1.1 15

Indian Bend Wash at Curry Road .29 .52 .75 1.2 3.6 1.1 .90 19

Box Culvert at 48th Street Drain .63 .79 1.2 1.6 2.3 1.2 .49 23

27th Avenue at Salt River .48 1.9 2.6 3.3 4.7 2.6 1.0 26

43rd Avenue and Peoria Avenue .35 .59 .95 1.5 3.9 1.3 1.0 25

Olive Avenue and 67th Avenue .57 .96 1.1 1.3 2.2 1.2 .39 19

91st Avenue WWTP outfall .61 1.5 2.7 4.6 7.2 3.1 2.0 24

Gila River at Buckeye Canal 2.2 3.4 4.3 5.1 6.9 4.3 1.2 40

Hassayampa River 2.0 6.0 7.5 12 17 8.8 3.7 48

Gila River at Gillespie Dam .10 6.6 8.6 11 25 8.6 4.4 293

Appendix 1. Statistical summaries of stream properties and water-chemistry constituents at surface-water quality monitoring stations in the 

Central Arizona Basins study area, through 1998—Continued

Station Minimum

Percentile

Maximum Mean

Standard 

deviation

Number of 

25th Median 75th samples
110 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Ammonia concentration, in mg/L as nitrogen

Central Highlands

Black River --- --- --- --- --- --- --- 0

White River --- --- --- --- --- --- --- 0

Pinal Creek <.02 .05 .13 .21 .41 --- --- 91

Salt River near Roosevelt <.01 <.01 .02 .04 .16 --- --- 156

Tonto Creek --- --- --- --- --- --- --- 0

Salt River below Stewart Mountain Dam <.01 <.01 .03 .05 .68 --- --- 117

Verde River near Clarkdale <.01 <.01 <.01 .02 .10 --- --- 105

Verde River near Cornville --- --- --- --- --- --- --- 0

Oak Creek near Sedona --- --- --- --- --- --- --- 0

Oak Creek at Red Rock Crossing <.01 <.01 <.01 .02 .05 --- --- 77

Oak Creek near Cornville --- --- --- --- --- --- --- 0

Verde River above West Clear Creek <.01 <.01 .02 .02 .04 --- --- 20

West Clear Creek <.01 <.01 .02 .02 .19 --- --- 39

Verde River near Camp Verde <.01 <.01 .02 .03 .10 --- --- 26

East Verde River <.01 <.01 <.01 .02 .19 --- --- 42

Wet Bottom Creek <.01 <.01 .02 .06 .16 --- --- 76

Verde River below Tangle Creek <.01 <.01 .02 .02 .26 --- --- 147

Verde River below Bartlett Dam <.01 .02 .04 .07 .47 --- --- 115

Turkey Creek <.01 .05 .10 .14 .69 --- --- 25

Agua Fria River near Rock Springs <.01 <.01 .02 .02 .09 --- --- 63

Agua Fria River below Waddell Dam <.01 .02 .03 .05 .24 --- --- 35

Basin and Range Lowlands

Central Arizona Project near Parker <0.01 <0.01 0.02 0.03 0.08 --- --- 39

Central Arizona Project at Phoenix <.01 <.01 .02 .02 1.4 --- --- 81

Gila River at Winkelman <.01 <.01 .02 .09 .11 --- --- 8

San Pedro River at Charleston <.01 <.01 .02 .03 .15 --- --- 83

San Pedro River below Aravaipa <.01 .06 .10 .13 .40 --- --- 35

San Pedro River at Winkelman <.01 <.01 .04 .06 .63 --- --- 16

Gila River at Kelvin <.01 .02 .02 .05 .13 --- --- 51

Santa Cruz River at Rio Rico .04 --- --- --- 2.8 --- --- 4

Santa Cruz River at Tubac .02 .12 1.1 2.3 6.1 1.7 1.9 27

Santa Cruz River at Cortaro 1.0 19 26 27 34 22 9.3 12

Santa Cruz River at Laveen .05 .08 .24 .46 1.4 .37 .40 14

Indian Bend Wash at Curry Road .02 .1 .23 .34 .92 --- --- 19

Box Culvert at 48th Street Drain .22 .76 1.1 1.5 2.4 1.1 .57 23

27th Avenue at Salt River .03 .33 .61 1.2 64 3.4 12 26

43rd Avenue and Peoria Avenue .68 1.2 1.7 2.4 7.8 2.2 1.6 25

Olive Avenue and 67th Avenue .19 .75 .97 1.4 3.4 1.1 .74 19

91st Avenue WWTP outfall 1.1 1.7 2.8 7.7 17 4.8 4.5 24

Gila River at Buckeye Canal .04 .82 1.7 4.0 15 3.1 3.7 40

Hassayampa River <.01 .02 .36 1.6 11 1.1 2.0 48

Gila River at Gillespie Dam <.01 .16 .89 3.2 11 2.1 2.6 203

Appendix 1. Statistical summaries of stream properties and water-chemistry constituents at surface-water quality monitoring stations in the 

Central Arizona Basins study area, through 1998—Continued

Station Minimum

Percentile

Maximum Mean

Standard 

deviation

Number of 

25th Median 75th samples
Appendix 1 111

Ammonia and organic nitrogen concentration, in mg/L as nitrogen

Central Highlands

Black River 0.04 0.15 0.19 0.34 1.5 0.28 0.25 43

White River .04 .16 .25 .55 4.7 .47 .72 43

Pinal Creek .12 .20 .40 .70 8.6 .54 .76 142

Salt River near Roosevelt <.01 .20 .30 .60 5.0 .51 .59 260

Tonto Creek .03 .11 .20 .60 4.8 .40 .66 53

Salt River below Stewart Mountain Dam .03 .27 .38 .60 2.9 .48 .40 145

Verde River near Clarkdale .03 .20 .20 .41 4.9 .40 .52 187

Verde River near Cornville .03 .15 .36 .44 .79 .33 .21 23

Oak Creek near Sedona .03 .16 .28 .43 2.3 .38 .47 22

Oak Creek at Red Rock Crossing <.02 .20 .30 .42 2.9 .41 .41 141

Oak Creek near Cornville <.01 .16 .22 .39 .67 .26 .18 29

Verde River above West Clear Creek .20 .20 .20 .28 .70 .29 .17 20

West Clear Creek .09 .17 .20 .20 1.5 .23 .22 39

Verde River near Camp Verde .12 .20 .40 .60 2.0 .47 .33 105

East Verde River .20 .20 .20 .20 .80 .24 .10 47

Wet Bottom Creek .20 .20 .32 .49 2.5 .41 .32 76

Verde River below Tangle Creek .10 .20 .30 .45 5.2 .43 .50 197

Verde River below Bartlett Dam .10 .28 .40 .60 11 .53 .92 144

Turkey Creek .32 .72 2.0 4.4 16 3.8 4.5 25

Agua Fria River near Rock Springs .20 .20 .20 .50 6.2 .52 .86 166

Agua Fria River below Waddell Dam .20 .40 .50 .60 1.4 .53 .25 75

Basin and Range Lowlands

Central Arizona Project near Parker 0.20 0.20 0.30 0.33 0.90 0.33 0.17 40

Central Arizona Project at Phoenix .20 .20 .30 .50 3.7 .44 .44 123

Gila River at Winkelman .10 .63 .85 1.1 11 1.3 1.7 81

San Pedro River at Charleston .10 .20 .20 .40 3.2 .35 .40 86

San Pedro River below Aravaipa .30 .49 .80 1.2 47 4.8 12 37

San Pedro River at Winkelman .10 .36 .64 2.8 81 6.2 15 34

Gila River at Kelvin .06 .42 .73 1.1 23 1.7 3.5 85

Santa Cruz River at Rio Rico 3.9 9.8 17 26 40 19 11 23

Santa Cruz River at Tubac .20 .51 3.0 5.5 21 3.9 4.3 26

Santa Cruz River at Cortaro 22 28 30 32 38 30 4.9 12

Santa Cruz River at Laveen .91 2.7 4.3 6.7 10 4.9 3.1 12

Indian Bend Wash at Curry Road .40 .91 1.2 1.8 2.5 1.3 .62 19

Box Culvert at 48th Street Drain .60 2.6 5.6 7.0 16 5.7 4.3 23

27th Avenue at Salt River 1.0 2.8 4.4 7.1 120 9.3 23 26

43rd Avenue and Peoria Avenue 1.3 3.1 3.9 5.3 12 4.7 2.9 25

Olive Avenue and 67th Avenue .90 2.6 3.4 4.1 11 3.7 2.3 18

91st Avenue WWTP outfall 2.2 3.0 4.4 10 20 7 5.6 24

Gila River at Buckeye Canal .84 2.0 3.0 6.0 18 4.8 4.3 40

Hassayampa River .84 1.5 2.0 3.6 15 3.2 3.0 48

Gila River at Gillespie Dam .20 2.0 3.2 5.2 17 4 2.8 244

Appendix 1. Statistical summaries of stream properties and water-chemistry constituents at surface-water quality monitoring stations in the 

Central Arizona Basins study area, through 1998—Continued

Station Minimum

Percentile

Maximum Mean

Standard 

deviation

Number of 

25th Median 75th samples
112 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Total nitrogen concentration, in mg/L as nitrogen

Central Highlands

Black River 0.01 0.17 0.22 0.36 1.9 0.31 0.30 43

White River .01 .18 .27 .57 4.7 .50 .74 43

Pinal Creek .11 .27 .50 .83 2.9 .66 .48 138

Salt River near Roosevelt .02 .28 .40 .70 4.0 .58 .56 256

Tonto Creek .01 .16 .33 .68 5.0 .47 .68 54

Salt River below Stewart Mountain Dam .05 .33 .46 .63 3.0 .55 .39 144

Verde River near Clarkdale .09 .29 .40 .60 5.4 .54 .56 178

Verde River near Cornville .09 .28 .42 .67 1.5 .51 .31 23

Oak Creek near Sedona .05 .18 .32 .49 2.4 .43 .48 22

Oak Creek at Red Rock Crossing .04 .30 .40 .56 3.0 .50 .42 131

Oak Creek near Cornville .04 .20 .29 .50 .76 .35 .22 29

Verde River above West Clear Creek .25 .28 .31 .42 .83 .39 .17 20

West Clear Creek .13 .22 .25 .29 1.5 .30 .23 39

Verde River near Camp Verde .15 .30 .50 .79 2.3 .59 .39 95

East Verde River .22 .22 .22 .30 .85 .28 .12 42

Wet Bottom Creek .25 .30 .40 .56 2.6 .49 .32 76

Verde River below Tangle Creek .12 .27 .40 .55 5.3 .52 .52 191

Verde River below Bartlett Dam .11 .38 .50 .71 11 .64 .91 144

Turkey Creek .50 .99 2.2 4.7 17 4.2 4.6 25

Agua Fria River near Rock Springs .25 .30 .40 .60 6.9 .66 .96 166

Agua Fria River below Waddell Dam .30 .46 .60 .72 1.5 .65 .26 75

Basin and Range Lowlands

Central Arizona Project near Parker 0.29 0.39 0.44 0.53 1.2 0.50 0.21 39

Central Arizona Project at Phoenix .25 .34 .40 .60 3.8 .54 .45 123

Gila River at Winkelman .12 .85 1.1 1.6 16 1.8 2.4 81

San Pedro River at Charleston .15 .25 .33 .55 3.4 .49 .46 81

San Pedro River below Aravaipa .81 1.2 1.4 1.8 48 5.5 12 37

San Pedro River at Winkelman .39 .91 1.5 3.0 85 7.0 16 34

Gila River at Kelvin .16 .51 .89 1.5 88 3.0 9.9 87

Santa Cruz River at Rio Rico 6.2 11 18 26 40 20 10 23

Santa Cruz River at Tubac .45 2.3 5.8 8.5 23 6.1 4.9 26

Santa Cruz River at Cortaro 23 28 30 33 39 30 4.7 12

Santa Cruz River at Laveen 2.0 4.4 5.8 8.3 12 6.4 3.1 12

Indian Bend Wash at Curry Road .75 1.8 2.3 2.4 5.6 2.4 1.2 19

Box Culvert at 48th Street Drain 1.7 3.4 6.5 8.3 18 7.0 4.6 23

27th Avenue at Salt River 1.9 5.4 7.2 9.7 120 12 22 26

43rd Avenue and Peoria Avenue 1.7 3.6 4.8 6.6 16 6.0 3.8 25

Olive Avenue and 67th Avenue 1.5 3.5 4.5 5.5 12 4.9 2.4 18

91st Avenue WWTP outfall 4.0 5.6 7.9 14 22 10 5.7 24

Gila River at Buckeye Canal 5.3 6.6 7.9 11 20 9.2 3.8 40

Hassayampa River 4.3 9.3 12 15 22 12 3.7 48

Gila River at Gillespie Dam .30 9.3 12 15 35 12 5.0 284

Appendix 1. Statistical summaries of stream properties and water-chemistry constituents at surface-water quality monitoring stations in the 

Central Arizona Basins study area, through 1998—Continued

Station Minimum

Percentile

Maximum Mean

Standard 

deviation

Number of 

25th Median 75th samples
Appendix 2 113

APPENDIX 2:

METHODS OF ESTIMATING STREAM LOADS AND BASIN INPUTS OF NUTRIENTS

Appendix 2 115

The explanatory variables allow for some of the 

variability of water-chemistry constituent concentrations to be accounted for as a result of the variability 

in streamflow, ln(Q/Q), ln(Q2/Q2), and Q0.5; season, 

sin(2πT) and cos(2πT); and long-term trends, T-T. 

Model diagnostics were used to select the explanatory 

variables significant to the regression model. Graphical 

diagnostics included normal probability plots of residuals and plots of residuals versus month, predicted values, flow, and time. P-values for beta coefficients, 

coefficient of determination (R2), and serial correlation 

coefficient also were provided. Explanatory variables 

with beta coefficients of p-values less than 0.10 were 

deemed significant and included in the model. Graphical diagnostics were then examined to ensure that there 

were no patterns in the relation between the residuals 

and flow, the residuals and predicted values, the residuals and season, and the residuals and time. 

The coefficient of determination (R2) for each waterchemistry constituent regression model represents the 

fraction of the variance in the water-chemistry constituent concentration that is explained by the regression 

model. For some of the water-chemistry constituents at 

some of the stations there were no significant regressors in the regression model except for the constant 

(β0); for these cases the R2 value is not reported. Daily 

stream loads were estimated by multiplying the mean 

daily streamflow for the day by the concentration estimated from the regression model. Daily stream loads 

then were aggregated into monthly and annual loads. 

The program ESTIMATOR provided estimates of 

the standard error of prediction (SEP) for the daily, 

monthly, and annual stream loads. The SEP is a measure of the precision of the stream load estimate and is 

useful when expressed as a percentage of the stream 

load estimate. Using the SEP, an approximate 95-percent confidence interval for the stream load estimate is:

,

where L is the annual load. In several cases the SEP 

was greater than 50 percent, which results in a 

95-percent confidence interval for the stream load 

that is equal to or greater than the estimate for the 

annual stream load. In these cases the uncertainty of 

the estimate was deemed too large to report annual 

stream load. 

L 1.96 SEP

100 × ---------- × L ⎝ ⎠ ⎛ ⎞ ±

Stream loads of dissolved solids and nutrients were estimated using the computer program ESTIMATOR 

(version 94.06). ESTIMATOR implements the minimum variance unbiased estimator described by Cohn and 

others (1989) and the adjusted maximum likelihood estimator described by Cohn (1988). The minimum variance 

unbiased estimator is superior to other methods, such as the traditional rating-curve method, because it does not 

produce bias that can be introduced when retransforming data from “log space,” where regression estimates are 

derived, to “real space,” where loads are calculated. The ESTIMATOR program calibrates a multiple linear regression model in the form of:

,

where

ln = the natural logarithm function;

C = the water-chemistry constituent concentration;

β0...β6 = the slope coefficients of the explanatory variables;

Q = the mean daily discharge for the day the sample was collected;

Q = the centered mean daily discharge;

T = time, in years, converted to decimal form;

T = the centered time, in years, converted to decimal form; and

e = the independent random error.

ln( ) C β0 β1

Q

Q

--- ⎝ ⎠ ⎛ ⎞ ln β2

Q2

Q2 ------

⎝ ⎠ ⎜ ⎟ ⎛ ⎞

ln β3Q0.5 β4 sin( ) 2πT β5 cos( ) 2πT β6 = + + ++ + + + ( ) T T – e
116 Assessment of Selected Inorganic Constituents in Streams in the CAZB Study Area, Arizona and Northern Mexico, through 1998

Estimates of inputs to basins from total (wet plus dry) 

annual atmospheric deposition of nitrogen (ammonia 

and nitrate) were determined using precipitationchemistry data from the National Atmospheric 

Deposition Program (NADP; National Atmospheric 

Deposition Program, 1998b). Data were collected in 

accordance with strict clean-handling procedures and 

sent to a central laboratory for analyses (National 

Atmospheric Deposition Program, 1998a). 

Precipitation-chemistry data for phosphorus were 

unavailable; therefore, estimates of phosphorus 

deposition were not made. Annual wet deposition 

inputs of ammonia and of nitrate for each basin were 

estimated as the mean of the concentrations from the 

three NADP precipitation chemistry sites in Arizona 

(figs. 3 and 7) for the period 1981–96, multiplied by the 

mean annual basin precipitation volume for the period 

1931–60 (estimated from basin-precipitation data by 

Garrett and Gellenbeck, 1991). No attempt was made 

to correct the annual mean input for the proximity of 

the NADP sites to the basins because data were 

available only from three NADP sites in Arizona, and 

these were about 80 kilometers outside of the study 

area (fig. 3). Estimates of dry deposition of nitrate were 

determined on the basis of wet deposition estimates of 

nitrate, and both wet and dry deposition estimates were 

adjusted for urban perturbations (Sisterson, 1990). 

Estimates of nutrient inputs to selected basins 

from fertilizer were determined using statewide sales 

data for 1995 (Arizona Agricultural Statistics Service, 

1996). State fertilizer sales (by mass) were assumed 

to equal State fertilizer use and were disaggregated 

into county fertilizer use on the basis of the number 

of farmland acres that fertilizer was applied to in each 

county (U.S. Department of Commerce, 1994). County 

fertilizer sales were apportioned to each basin on the 

basis of county agricultural land use (digital data modified from Anderson and others, 1976; unpublished digital data from Maricopa County Association of 

Governments, Pima County, and the University of Arizona). 

 Estimates of nutrient inputs to selected basins 

from bodily waste from humans were determined on 

the basis of human populations (data for locations in 

Arizona from Hitt, 1994; data for locations in Mexico, 

from Lorey, 1990) and a per capita nutrient coefficient 

(U.S. Environmental Protection Agency, 1980). Human 

populations within each basin were estimated by aggregating populations within census tracts for those areas 

in the United States, and adding in populations by city 

for those locations in Mexico. Nutrient inputs from 

bodily waste from humans were subcategorized as that 

to be treated by a wastewater-treatment plant, or that to 

be treated by a septic system, on the basis of plumbing 

characteristics data (available with precision to the 

Census Designated Place; U.S. Department of Commerce, 1992). Septic tanks were considered nonpoint 

sources of pollution, whereas wastewater-treatment 

plants were considered point sources. 

Estimates of nutrient inputs to selected basins were 

determined for bodily waste from livestock on feed 

(dairy cows, beef cattle, hogs, and horses). Nutrient 

inputs from grazing livestock were not estimated in 

this study because these nutrients were considered to 

be a conversion of one form to another with no net 

gain of nutrients to the basin; in contrast, fed livestock 

consume nutrients in their food, which is either 

imported to the basin or is generated specifically for 

livestock consumption, and thus results in a net 

increase in nutrients. Nutrient inputs for bodily waste 

from livestock on feed were estimated for each county 

on the basis of county livestock populations (Arizona 

Agricultural Statistics Service, 1997; U.S. Department 

of Commerce, 1994) and nutrient coefficients (U.S. 

Department of Agriculture, 1992). The nutrient inputs 

for each county were then apportioned to each basin on 

the basis of county agricultural land use (digital data 

modified from Anderson and others, 1976; unpublished 

digital data from Maricopa County Association of 

Governments, Pima County, and the University of 

Arizona). 

In the case of the middle Gila River Basin and

the Hassayampa River Basin, some nutrients are

transported into the basin by surface water. For the 

middle Gila River Basin, nutrients enter the upstream 

end of the basin through the Gila River at Kelvin and 

also in the western side of the basin through the Central 

Arizona Project Canal. For the Hassayampa River 

Basin, nutrients enter through the Buckeye Canal in the 

lower part of the basin. The stream loads of nutrients 

carried by the streams and canals entering these basins 

are used as the estimates for the basin inputs from 

surface-water flow. 

Estimates of nutrient inputs to selected basins from 

industrial sources were determined using data from the 

toxic chemical release inventory (Arizona Department 

of Environmental Quality, 1997). Nutrient inputs only 

include those releases that are toxic and are required to 

be reported by the U.S. Environmental Protection 

Agency, and thus are likely to be underestimated. 

Estimates from site-specific data were aggregated into 

basin estimates. 
Anning—ASSESSMENT OF SELECTED INORGANIC CONSTITUENTS IN STREAMS IN THE

CENTRAL ARIZONA BASINS STUDY AREA, ARIZONA AND NORTHERN MEXICO, THROUGH 1998—

U.S. Geological Survey Water-Resources Investigations Report 03—4063